bio-integrity: Fix use of bs->bio_integrity_pool after free

This fixes a copy and paste error introduced by 9f060e22
("block: Convert integrity to bvec_alloc_bs()").

Found by Coverity (CID 1020654).

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kent Overstreet <koverstreet@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>