loop/usb/vhost-scsi/xen-scsiback: Fix use of __transport_register_session

This patch changes loopback, usb-gadget, vhost-scsi and xen-scsiback
fabric code to invoke transport_register_session() instead of the
unprotected flavour, to ensure se_tpg->session_lock is taken when
adding new session list nodes to se_tpg->tpg_sess_list.

Note that since these four fabric drivers already hold their own
internal TPG mutexes when accessing se_tpg->tpg_sess_list, and
consist of a single se_session created through configfs attribute
access, no list corruption can currently occur.

So for correctness sake, go ahead and use the se_tpg->session_lock
protected version for these four fabric drivers.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/xen/xen-scsiback.c

@@ -1661,11 +1661,8 @@ static int scsiback_make_nexus(struct scsiback_tpg *tpg,
 			 name);
 		goto out;
 	}
-	/*
-	 * Now register the TCM pvscsi virtual I_T Nexus as active with the
-	 * call to __transport_register_session()
-	 */
-	__transport_register_session(se_tpg, tv_nexus->tvn_se_sess->se_node_acl,
+	/* Now register the TCM pvscsi virtual I_T Nexus as active. */
+	transport_register_session(se_tpg, tv_nexus->tvn_se_sess->se_node_acl,
 			tv_nexus->tvn_se_sess, tv_nexus);
 	tpg->tpg_nexus = tv_nexus;