IB/isert: fix T10-pi check mask setting

A copy/paste bug (probably) caused setting of an app_tag check mask
in case where a ref_tag check was needed.

Fixes: 38a2d0d4 ("IB/isert: convert to the generic RDMA READ/WRITE API")
Fixes: 9e961ae7 ("IB/isert: Support T10-PI protected transactions")
Cc: stable@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>