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>