Add option for forcible unmounting dataset while receiving snapshot.

Currently when the dataset is in use we can't receive snapshots.

    zfs send test/1@asd | zfs recv -FM test/2
    cannot unmount '/test/2': Device busy

This commits add option 'M' which attempts to forcibly unmount the
dataset.  Thanks to this we can enforce receiving snapshots in a
single step.

Note that this functionality is not supported on Linux because the
VFS will prevent active mounted filesystems from being unmounted,
even with the force option.  This is the intended VFS behavior.

Test cases were added to verify the expected behavior based on
the platform.

Discussed-with: Pawel Jakub Dawidek <pjd@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
External-issue: https://reviews.freebsd.org/D22306
Closes #9904

tests/runfiles/common.run

@@ -208,9 +208,10 @@ tests = ['zfs_receive_001_pos', 'zfs_receive_002_pos', 'zfs_receive_003_pos',
     'zfs_receive_007_neg', 'zfs_receive_008_pos', 'zfs_receive_009_neg',
     'zfs_receive_010_pos', 'zfs_receive_011_pos', 'zfs_receive_012_pos',
     'zfs_receive_013_pos', 'zfs_receive_014_pos', 'zfs_receive_015_pos',
-    'receive-o-x_props_override', 'zfs_receive_from_encrypted',
-    'zfs_receive_to_encrypted', 'zfs_receive_raw',
-    'zfs_receive_raw_incremental', 'zfs_receive_-e', 'zfs_receive_raw_-d']
+    'zfs_receive_016_pos', 'receive-o-x_props_override',
+    'zfs_receive_from_encrypted', 'zfs_receive_to_encrypted',
+    'zfs_receive_raw', 'zfs_receive_raw_incremental', 'zfs_receive_-e',
+    'zfs_receive_raw_-d']
 tags = ['functional', 'cli_root', 'zfs_receive']
 
 [tests/functional/cli_root/zfs_rename]

tests/zfs-tests/tests/functional/cli_root/zfs_receive/Makefile.am

@@ -17,6 +17,7 @@ dist_pkgdata_SCRIPTS = \
 	zfs_receive_013_pos.ksh \
 	zfs_receive_014_pos.ksh \
 	zfs_receive_015_pos.ksh \
+	zfs_receive_016_pos.ksh \
 	receive-o-x_props_override.ksh \
 	zfs_receive_from_encrypted.ksh \
 	zfs_receive_to_encrypted.ksh \

tests/zfs-tests/tests/functional/cli_root/zfs_receive/zfs_receive_016_pos.ksh

@@ -0,0 +1,85 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+# Copyright (c) 2020 by Mariusz Zaborski <oshogbo@FreeBSD.org>.
+
+#
+# DESCRIPTION:
+# Verify 'zfs recv' can forcibly unmount filesystem while receiving
+# stream.
+#
+# STRATEGY:
+# 1. Create snapshot of file system
+# 2. Make a zfs filesystem mountpoint busy
+# 3. Receive filesystem with force flag.
+# 4. Verify that stream was received or failed on Linux.
+#
+
+. $STF_SUITE/tests/functional/cli_root/cli_common.kshlib
+
+verify_runnable "both"
+
+function cleanup
+{
+	cd $curpath
+
+	for snap in $init_snap $rst_snap; do
+                snapexists $snap && \
+                        destroy_snapshot $snap
+        done
+
+	datasetexists $rst_root && \
+		destroy_dataset $rst_root
+
+	for file in $full_bkup
+	do
+		[[ -e $file ]] && \
+			log_must rm -f $file
+	done
+
+	[[ -d $TESTDIR1 ]] && \
+		log_must rm -rf $TESTDIR1
+}
+
+log_assert "Verify 'zfs recv' can forcibly unmount busy filesystem."
+log_onexit cleanup
+
+curpath=`dirname $0`
+init_snap=$TESTPOOL/$TESTFS@init_snap
+full_bkup=$TEST_BASE_DIR/fullbkup.$$
+rst_root=$TESTPOOL/rst_ctr
+rst_snap=$rst_root@init_snap
+
+log_note "Verify 'zfs recv' can forcible unmount busy filesystem."
+
+# Preparation
+log_must zfs create $rst_root
+[[ ! -d $TESTDIR1 ]] && \
+	log_must mkdir -p $TESTDIR1
+log_must zfs set mountpoint=$TESTDIR1 $rst_root
+
+log_must zfs snapshot $init_snap
+log_must eval "zfs send $init_snap > $full_bkup"
+
+# Test
+log_must cd $TESTDIR1
+if is_linux; then
+    # Linux does not support it.
+    log_mustnot zfs receive -MF $rst_snap < $full_bkup
+else
+    log_must zfs receive -MF $rst_snap < $full_bkup
+fi
+
+log_pass "The busy filesystem was unmounted or busy as expected."