dm integrity: make dm_integrity_init and dm_integrity_exit static

Fix sparse warnings:

drivers/md/dm-integrity.c:3619:12: warning:
 symbol 'dm_integrity_init' was not declared. Should it be static?
drivers/md/dm-integrity.c:3638:6: warning:
 symbol 'dm_integrity_exit' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>