[iscsi] Ensure ISID is consistent within an iSCSI session

Commit 5f4ab0d ("[iscsi] Randomise a portion of the ISID to force new
session instantiation") introduced a regression by randomising the
ISID on each call to iscsi_start_login(), which may be called more
than once per connection, rather than on each call to
iscsi_open_connection(), which is guaranteed to be called only once
per connection.  This is incorrect behaviour that causes our
connection to be rejected by some iSCSI targets (observed with a
COMSTAR target under OpenSolaris).

Fix by generating the ISID in iscsi_open_connection(), and storing the
randomised ISID as part of the session state.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/iscsi.h

@@ -539,6 +539,12 @@ struct iscsi_session {
 	/** CHAP response (used for both initiator and target auth) */
 	struct chap_response chap;
 
+	/** Initiator session ID (IANA format) qualifier
+	 *
+	 * This is part of the ISID.  It is generated randomly
+	 * whenever a new connection is opened.
+	 */
+	uint16_t isid_iana_qual;
 	/** Initiator task tag
 	 *
 	 * This is the tag of the current command.  It is incremented