scsi: RDMA/iser: Use scsi_cmd_to_rq() instead of scsi_cmnd.request

Prepare for removal of the request pointer by using scsi_cmd_to_rq()
instead. This patch does not change any functionality.

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Max Gurtovoy <[e-mail]>
Reviewed-by: Sagi Grimberg <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Bart Van Assche <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/infiniband/ulp/iser/iser_memory.c

···
 {
 	domain->sig_type = IB_SIG_TYPE_T10_DIF;
 	domain->sig.dif.pi_interval = scsi_prot_interval(sc);
-	domain->sig.dif.ref_tag = t10_pi_ref_tag(sc->request);
+	domain->sig.dif.ref_tag = t10_pi_ref_tag(scsi_cmd_to_rq(sc));
 	/*
 	 * At the moment we hard code those, but in the future
 	 * we will take them from sc.