block: Fix merge logic when CONFIG_BLK_DEV_INTEGRITY is not defined

Commit 4eaf99beadce switched to returning bool and as a result reversed
the logic of the integrity merge checks.  However, the empty stubs used
when the block integrity code is compiled out were still returning
0. Make these stubs return "true".

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reported-by: Michael L. Semon <mlsemon35@gmail.com>
Tested-by: Michael L. Semon <mlsemon35@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>