Stack overflow when destroying deeply nested clones

Destroy operations on deeply nested chains of clones can overflow
the stack:

        Depth    Size   Location    (221 entries)
        -----    ----   --------
  0)    15664      48   mutex_lock+0x5/0x30
  1)    15616       8   mutex_lock+0x5/0x30
...
 26)    13576      72   dsl_dataset_remove_clones_key.isra.4+0x124/0x1e0 [zfs]
 27)    13504      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
 28)    13432      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
...
185)     2128      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
186)     2056      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
187)     1984      72   dsl_dataset_remove_clones_key.isra.4+0x18a/0x1e0 [zfs]
188)     1912     136   dsl_destroy_snapshot_sync_impl+0x4e0/0x1090 [zfs]
189)     1776      16   dsl_destroy_snapshot_check+0x0/0x90 [zfs]
...
218)      304     128   kthread+0xdf/0x100
219)      176      48   ret_from_fork+0x22/0x40
220)      128     128   kthread+0x0/0x100

Fix this issue by converting dsl_dataset_remove_clones_key() from
recursive to iterative.

Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7279 
Closes #7810

tests/runfiles/linux.run

@@ -120,7 +120,7 @@ tags = ['functional', 'cli_root', 'zfs_change-key']
 tests = ['zfs_clone_001_neg', 'zfs_clone_002_pos', 'zfs_clone_003_pos',
     'zfs_clone_004_pos', 'zfs_clone_005_pos', 'zfs_clone_006_pos',
     'zfs_clone_007_pos', 'zfs_clone_008_neg', 'zfs_clone_009_neg',
-    'zfs_clone_010_pos', 'zfs_clone_encrypted']
+    'zfs_clone_010_pos', 'zfs_clone_encrypted', 'zfs_clone_deeply_nested']
 tags = ['functional', 'cli_root', 'zfs_clone']
 
 [tests/functional/cli_root/zfs_copies]

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

@@ -12,4 +12,5 @@ dist_pkgdata_SCRIPTS = \
 	zfs_clone_008_neg.ksh \
 	zfs_clone_009_neg.ksh \
 	zfs_clone_010_pos.ksh \
-	zfs_clone_encrypted.ksh
+	zfs_clone_encrypted.ksh \
+	zfs_clone_deeply_nested.ksh

tests/zfs-tests/tests/functional/cli_root/zfs_clone/zfs_clone_deeply_nested.ksh

@@ -0,0 +1,69 @@
+#!/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 2018, loli10K <ezomori.nozomu@gmail.com>. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION:
+# Deeply nested clones can be created and destroyed successfully
+#
+# STRATEGY:
+# 1. Create a deeply nested chain of clones
+# 2. Verify we can promote and destroy datasets in the chain without issues
+# NOTE:
+# Ported from scripts used to reproduce issue #3959 and #7279
+#
+
+verify_runnable "both"
+
+function cleanup
+{
+	destroy_dataset "$clonesfs" "-rRf"
+}
+log_onexit cleanup
+
+log_assert "Deeply nested clones should be created and destroyed without issues"
+
+snapname='snap'
+snaprename='temporary-snap'
+clonesfs="$TESTPOOL/$TESTFS1"
+
+# NOTE: set mountpoint=none to avoid mount/umount calls and speed up the process
+log_must zfs create -o mountpoint=none $clonesfs
+log_must zfs create $clonesfs/0
+dsname="$clonesfs/0@$snapname"
+log_must zfs snapshot $dsname
+
+# 1. Create a deeply nested chain of clones
+for c in {1..250}; do
+	log_must zfs clone $dsname $clonesfs/$c
+	dsname="$clonesfs/$c@$snapname"
+	log_must zfs snapshot $dsname
+done
+
+# 2. Verify we can promote and destroy datasets in the chain without issues
+for c in {0..249}; do
+	log_must zfs rename $clonesfs/$c@$snapname $clonesfs/$c@$snaprename
+	log_must zfs promote $clonesfs/$((c+1))
+	log_must zfs destroy -r $clonesfs/$c
+	log_must zfs destroy $clonesfs/$((c+1))@$snaprename
+done
+
+log_pass "Deeply nested clones can be created and destroyed successfully"