dm snapshot: use barrier when writing exception store

Send barrier requests when updating the exception area.

Exception area updates need to be ordered w.r.t. data writes, so that
the writes are not reordered in hardware disk cache.
Signed-off-by: NMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

drivers/md/dm-snap-persistent.c

@@ -636,7 +636,7 @@ static void persistent_commit_exception(struct dm_exception_store *store,
 	/*
 	 * Commit exceptions to disk.
 	 */
-	if (ps->valid && area_io(ps, WRITE))
+	if (ps->valid && area_io(ps, WRITE_BARRIER))
 		ps->valid = 0;
 
 	/*