[SCSI] zfcp: fix mempool usage for status_read requests

When allocating fsf requests without qtcb, store the pointer to the
mempool in the fsf requests for later call to mempool_free. This
codepath is only used by the status_read requests.

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>