block: remove useless assignment in bio_split

Remove useless assignment to the variable "split" because the variable is
unconditionally assigned later.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -1819,7 +1819,7 @@ EXPORT_SYMBOL(bio_endio);
 struct bio *bio_split(struct bio *bio, int sectors,
 		      gfp_t gfp, struct bio_set *bs)
 {
-	struct bio *split = NULL;
+	struct bio *split;
 
 	BUG_ON(sectors <= 0);
 	BUG_ON(sectors >= bio_sectors(bio));