[SCSI] zfcp: Acquire qdio_stat_lock when reading the queue utilization

req_q_util is not atomic, so the qdio_stat_lock must be held when
reading this variable.

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