iscsi: Resolve iscsi_proto.h naming conflicts with drivers/target/iscsi

This patch renames the following iscsi_proto.h structures to avoid
namespace issues with drivers/target/iscsi/iscsi_target_core.h:

*) struct iscsi_cmd -> struct iscsi_scsi_req
*) struct iscsi_cmd_rsp -> struct iscsi_scsi_rsp
*) struct iscsi_login -> struct iscsi_login_req

This patch includes useful ISCSI_FLAG_LOGIN_[CURRENT,NEXT]_STAGE*,
and ISCSI_FLAG_SNACK_TYPE_* definitions used by iscsi_target_mod, and
fixes the incorrect definition of struct iscsi_snack to following
RFC-3720 Section 10.16. SNACK Request.

Also, this patch updates libiscsi, iSER, be2iscsi, and bn2xi to
use the updated structure definitions in a handful of locations.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
diff --git a/drivers/scsi/be2iscsi/be_main.h b/drivers/scsi/be2iscsi/be_main.h
index 081c171..5ce5170 100644
--- a/drivers/scsi/be2iscsi/be_main.h
+++ b/drivers/scsi/be2iscsi/be_main.h
@@ -397,7 +397,7 @@
 };
 
 struct be_cmd_bhs {
-	struct iscsi_cmd iscsi_hdr;
+	struct iscsi_scsi_req iscsi_hdr;
 	unsigned char pad1[16];
 	struct pdu_data_out iscsi_data_pdu;
 	unsigned char pad2[BE_SENSE_INFO_SIZE -
@@ -428,7 +428,7 @@
 };
 
 struct be_status_bhs {
-	struct iscsi_cmd iscsi_hdr;
+	struct iscsi_scsi_req iscsi_hdr;
 	unsigned char pad1[16];
 	/**
 	 * The plus 2 below is to hold the sense info length that gets