Introduce a destroy_dataset helper

Datasets can be busy when calling zfs destroy. Introduce
a helper function to destroy datasets and use it to destroy
datasets in zfs_allow_004_pos, zfs_promote_008_pos, and
zfs_destroy_002_pos.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #7224 
Closes #7246 
Closes #7249 
Closes #7267

tests/zfs-tests/tests/functional/cli_root/zfs_destroy/zfs_destroy_002_pos.ksh

@@ -49,8 +49,7 @@ function cleanup
 {
 	typeset -i i=0
 	while (( $i < ${#data_objs[*]} )); do
-		datasetexists "${data_objs[i]}" && \
-			zfs destroy -rf ${data_objs[i]}
+		destroy_dataset "${data_objs[i]}" "-rf"
 		((i = i + 1))
 	done
 }
@@ -87,7 +86,7 @@ done
 
 i=0
 while (( $i < ${#data_objs[*]} )); do
-	log_must zfs destroy ${data_objs[i]}
+	destroy_dataset "${data_objs[i]}"
 	datasetexists ${data_objs[i]} && \
 		log_fail "'zfs destroy <filesystem>|<volume>|<snapshot>' fail."
 	((i = i + 1))

tests/zfs-tests/tests/functional/cli_root/zfs_promote/zfs_promote_008_pos.ksh

@@ -49,7 +49,7 @@ function cleanup
 		log_must zfs promote $vol
 	fi
 
-	log_must zfs destroy -rR $snap
+	destroy_dataset "$snap" "-rR"
 }
 
 log_assert "'zfs promote' can promote a volume clone."

tests/zfs-tests/tests/functional/delegate/delegate_common.kshlib

@@ -53,15 +53,11 @@ function cleanup_user_group
 #
 function restore_root_datasets
 {
-	if datasetexists $ROOT_TESTFS ; then
-		log_must zfs destroy -Rf $ROOT_TESTFS
-	fi
+	destroy_dataset "$ROOT_TESTFS" "-Rf"
 	log_must zfs create $ROOT_TESTFS
 
 	if is_global_zone ; then
-		if datasetexists $ROOT_TESTVOL ; then
-			log_must zfs destroy -Rf $ROOT_TESTVOL
-		fi
+		destroy_dataset "$ROOT_TESTVOL" "-Rf"
 		log_must zfs create -V $VOLSIZE $ROOT_TESTVOL
 		block_device_wait
 	fi