dm verity: make verity_for_io_block static

Fixes the following sparse warning:

drivers/md/dm-verity-target.c:375:6: warning:
 symbol 'verity_for_io_block' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-verity-target.c

@@ -348,8 +348,8 @@ out:
 /*
  * Calculates the digest for the given bio
  */
-int verity_for_io_block(struct dm_verity *v, struct dm_verity_io *io,
-			struct bvec_iter *iter, struct crypto_wait *wait)
+static int verity_for_io_block(struct dm_verity *v, struct dm_verity_io *io,
+			       struct bvec_iter *iter, struct crypto_wait *wait)
 {
 	unsigned int todo = 1 << v->data_dev_block_bits;
 	struct bio *bio = dm_bio_from_per_bio_data(io, v->ti->per_io_data_size);