dm writecache: add unlikely for getting two block with same LBA

In function writecache_writeback, entries g and f has same original
sector only happens at entry f has been committed, but entry g has
NOT yet.

The probability of this happening is very low in the following
256 blocks at most of entry e.

Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -1628,8 +1628,8 @@ restart:
 			if (unlikely(!next_node))
 				break;
 			g = container_of(next_node, struct wc_entry, rb_node);
-			if (read_original_sector(wc, g) ==
-			    read_original_sector(wc, f)) {
+			if (unlikely(read_original_sector(wc, g) ==
+			    read_original_sector(wc, f))) {
 				f = g;
 				continue;
 			}