PDU use serial32 arithmetic for cmdsn, maxcmdsn and expcmdsn.

RFC3720 says that cmdsn comparison must be done using
serial32 arithmetic. This will definetly avoid a deadlock
if cmdsn wraps from 2^32-1 to 0.

Signed-off-by: Peter Lieven <pl@kamp.de>

include/iscsi-private.h

@@ -325,6 +325,9 @@ iscsi_log_message(struct iscsi_context *iscsi, int level, const char *format, ..
 void
 iscsi_add_to_outqueue(struct iscsi_context *iscsi, struct iscsi_pdu *pdu);
 
+int
+iscsi_serial32_compare(u_int32_t s1, u_int32_t s2);
+
 #ifdef __cplusplus
 }
 #endif