Fix log vdev removal issues

When we clear the log, we should clear all the fields, not only
zh_log.  Otherwise remaining ZIL_REPLAY_NEEDED will prevent the
vdev removal.  Handle it also from the other side, when zh_log
is already cleared, while zh_flags is not.

spa_vdev_remove_log() asserts that allocated space on removed log
device is zero.  While it should be so in perfect world, it might
be not if space leaked at any point.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #18277

tests/zfs-tests/tests/Makefile.am

@@ -1947,6 +1947,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/removal/removal_with_faulted.ksh \
 	functional/removal/removal_with_ganging.ksh \
 	functional/removal/removal_with_hole.ksh \
+	functional/removal/removal_with_missing_log.ksh \
 	functional/removal/removal_with_indirect.ksh \
 	functional/removal/removal_with_remove.ksh \
 	functional/removal/removal_with_scrub.ksh \

tests/zfs-tests/tests/functional/removal/removal_with_missing_log.ksh

@@ -0,0 +1,95 @@
+#!/bin/ksh -p
+# SPDX-License-Identifier: CDDL-1.0
+#
+# 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) 2026, TrueNAS.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/removal/removal.kshlib
+
+#
+# DESCRIPTION:
+#	Verify that a missing SLOG device can be removed even when
+#	ZIL blocks exist on it.
+#
+# STRATEGY:
+#	1. Create a pool with a SLOG device
+#	2. Freeze the pool and write data to ZIL
+#	3. Export the pool (ZIL blocks remain uncommitted)
+#	4. Import with -N to claim logs without replay
+#	5. Export and clear SLOG device labels to simulate failure
+#	6. Import with -m (missing devices allowed)
+#	7. Remove the missing SLOG vdev
+#	8. Verify pool is healthy and space accounting is correct
+#
+
+verify_runnable "global"
+
+log_assert "Removal of missing SLOG with ZIL blocks succeeds"
+
+function cleanup
+{
+	poolexists $TESTPOOL && destroy_pool $TESTPOOL
+}
+
+log_onexit cleanup
+
+VDEV1="$(echo $DISKS | cut -d' ' -f1)"
+VDEV2="$(echo $DISKS | cut -d' ' -f2)"
+
+# Create pool with SLOG and dataset
+log_must zpool create $TESTPOOL $VDEV1 log $VDEV2
+log_must zfs create $TESTPOOL/$TESTFS
+
+# Create initial ZIL header (required before freezing)
+log_must dd if=/dev/zero of=/$TESTPOOL/$TESTFS/init \
+    conv=fdatasync,fsync bs=1 count=1
+
+# Freeze pool and write data to ZIL
+log_must zpool freeze $TESTPOOL
+log_must dd if=/dev/urandom of=/$TESTPOOL/$TESTFS/file1 \
+    oflag=sync bs=128k count=128
+
+# Export with uncommitted ZIL transactions
+log_must zpool export $TESTPOOL
+
+# Import with -N to claim logs without mounting/replaying
+log_must zpool import -N $TESTPOOL
+log_must zpool export $TESTPOOL
+
+# Clear SLOG labels to simulate device failure
+log_must zpool labelclear -f $VDEV2
+
+# Import with missing SLOG allowed
+log_must zpool import -m $TESTPOOL
+log_must eval "zpool status $TESTPOOL | grep UNAVAIL"
+
+# Remove the missing SLOG - should succeed
+log_must zpool remove $TESTPOOL $VDEV2
+log_must zpool wait -t remove $TESTPOOL
+sync_pool $TESTPOOL
+log_mustnot eval "zpool status -v $TESTPOOL | grep $VDEV2"
+
+# Verify pool health
+log_must zpool scrub -w $TESTPOOL
+log_must check_pool_status $TESTPOOL "errors" "No known data errors"
+
+# Verify space accounting is correct
+log_must zdb -c $TESTPOOL
+
+log_pass "Removal of missing SLOG with ZIL blocks succeeded"