Fixes gcc '-Wunused-but-set-variable' warning:

drivers/md/dm-integrity.c: In function 'integrity_metadata':
drivers/md/dm-integrity.c:1557:12: warning:
 variable 'save_metadata_offset' set but not used [-Wunused-but-set-variable]
drivers/md/dm-integrity.c:1556:12: warning:
 variable 'save_metadata_block' set but not used [-Wunused-but-set-variable]

They are never used, so remove it.
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>