block: fix bugs in bio-integrity mempool usage

Fix two bugs in the bio integrity code:

 use_bip_pool() always returns 0 because it checks against the wrong limit,
 causing the mempool to be used only when regular allocation fails.

 When the mempool is used as a fallback we don't free the data properly.

Signed-Off-By: Chuck Ebbert <cebbert@redhat.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>