zfs_mount_all_mountpoints: cleanup_all should leave pool root mounted

if pool root is not mounted, then zpool umount in next test will leave
dataset mountpoint directory around and next zfs mount -a will fail
with error: cannot mount '/testpool': directory is not empty

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Toomas Soome <[email protected]>
Closes #11417

Author: tsoome

tests/zfs-tests/tests/functional/cli_root/zfs_mount/zfs_mount_all_mountpoints.ksh

@@ -109,6 +109,8 @@ function cleanup_all
 	export __ZFS_POOL_RESTRICT="$TESTPOOL"
 	log_must zfs $unmountall
 	unset __ZFS_POOL_RESTRICT
+	# make sure we leave $TESTPOOL mounted
+	log_must zfs mount $TESTPOOL
 
 	for fs in ${filesystems[@]}; do
 		cleanup_filesystem "$TESTPOOL" "$fs"