dm writecache: handle REQ_FUA

Call writecache_flush() on REQ_FUA in writecache_map().

Cc: stable@vger.kernel.org # 4.18+
Signed-off-by: Maged Mokhtar <mmokhtar@petasan.org>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -1218,7 +1218,8 @@ bio_copy:
 			}
 		} while (bio->bi_iter.bi_size);
 
-		if (unlikely(wc->uncommitted_blocks >= wc->autocommit_blocks))
+		if (unlikely(bio->bi_opf & REQ_FUA ||
+			     wc->uncommitted_blocks >= wc->autocommit_blocks))
 			writecache_flush(wc);
 		else
 			writecache_schedule_autocommit(wc);