fs/bio-integrity: remove duplicate code

Most code of function bio_integrity_verify and bio_integrity_generate
is the same, so introduce a help function bio_integrity_generate_verify()
to remove the duplicate code.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jens Axboe <axboe@fb.com>