[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

 	/** 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

src/net/tcp/iscsi.c

 	if ( iscsi->target_username )
 		iscsi->status |= ISCSI_STATUS_AUTH_REVERSE_REQUIRED;
 
+	/* Assign new ISID */
+	iscsi->isid_iana_qual = ( random() & 0xffff );
+
 	/* Assign fresh initiator task tag */
 	iscsi_new_itt ( iscsi );
 
 	ISCSI_SET_LENGTHS ( request->lengths, 0, len );
 	request->isid_iana_en = htonl ( ISCSI_ISID_IANA |
 					IANA_EN_FEN_SYSTEMS );
-	request->isid_iana_qual = ( random() & 0xffff );
+	request->isid_iana_qual = htons ( iscsi->isid_iana_qual );
 	/* tsih left as zero */
 	request->itt = htonl ( iscsi->itt );
 	/* cid left as zero */