ZTS: avoid race to unmount in zfs_rollback_001

The zfs_rollback_001 test modifies files in a temporary, test dataset
repeatedly.  Before each iteration, any preexisting dataset is removed,
after unmounted with umount -f, if necessary.

Add a short delay after the forced unmount, avoiding a race that can
prevent zfs destroy from succeeding, leading to a test failure.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <aerusso@aerusso.net>
Closes openzfs#11451

tests/zfs-tests/tests/functional/cli_root/zfs_rollback/zfs_rollback_001_pos.ksh

@@ -80,6 +80,7 @@ function test_n_check #opt num_snap_clone num_rollback
 	if datasetexists $VOL; then
 		if ismounted $TESTDIR1 $NEWFS_DEFAULT_FS; then
 			log_must umount -f $TESTDIR1
+			sleep 0.1
 		fi
 
 		log_must zfs destroy -Rf $VOL