bcache: don't pass BIOSET_NEED_BVECS for the 'bio_set' embedded in 'cache_set'

This bioset is just for allocating bio only from bio_next_split, and it
needn't bvecs, so remove the flag.

Cc: linux-bcache@vger.kernel.org
Cc: Coly Li <colyli@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

drivers/md/bcache/super.c

@@ -1939,7 +1939,7 @@ struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)
 		goto err;
 
 	if (bioset_init(&c->bio_split, 4, offsetof(struct bbio, bio),
-			BIOSET_NEED_BVECS|BIOSET_NEED_RESCUER))
+			BIOSET_NEED_RESCUER))
 		goto err;
 
 	c->uuids = alloc_meta_bucket_pages(GFP_KERNEL, sb);