scsi: libiscsi: Annotate locking assumptions

This patch avoids that sparse reports the following:

drivers/scsi/libiscsi.c:1844:23: warning: context imbalance in 'iscsi_exec_task_mgmt_fn' - unexpected unlock
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libiscsi.c

@@ -1832,6 +1832,7 @@ static void iscsi_tmf_timedout(struct timer_list *t)
 static int iscsi_exec_task_mgmt_fn(struct iscsi_conn *conn,
 				   struct iscsi_tm *hdr, int age,
 				   int timeout)
+	__must_hold(&session->frwd_lock)
 {
 	struct iscsi_session *session = conn->session;
 	struct iscsi_task *task;