bio-integrity: cleanup adding integrity pages to bip's bvec.

bio_integrity_add_page() returns the add length if successful, else 0,
just as bio_add_page.  Simply check return value checking in
bio_integrity_prep to not deal with a > 0 but < len case that can't
happen.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Martin K. Petersen <martin.petersen@oracle.com>

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jinyoung Choi <j-young.choi@samsung.com>
Tested-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Reviewed-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20230803025058epcms2p5a4d0db5da2ad967668932d463661c633@epcms2p5
Signed-off-by: Jens Axboe <axboe@kernel.dk>