iscsi-target: Expose per endpoint dynamic_sessions attribute

This patch exposes a new ../iscsi/$IQN/$TPGT/dynamic_sessions attribute
to dump the currently active sessions by iSCSI InitiatorName that have
been created with dynamically generated se_node_acls.

This information is useful so that user-space can optionally perform
dynamic -> explicit NodeACL conversion based on $INITIATOR_WWPN.
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_configfs.c

@@ -1410,8 +1410,18 @@ static ssize_t lio_target_tpg_store_enable(
 
 TF_TPG_BASE_ATTR(lio_target, enable, S_IRUGO | S_IWUSR);
 
+static ssize_t lio_target_tpg_show_dynamic_sessions(
+	struct se_portal_group *se_tpg,
+	char *page)
+{
+	return target_show_dynamic_sessions(se_tpg, page);
+}
+
+TF_TPG_BASE_ATTR_RO(lio_target, dynamic_sessions);
+
 static struct configfs_attribute *lio_target_tpg_attrs[] = {
 	&lio_target_tpg_enable.attr,
+	&lio_target_tpg_dynamic_sessions.attr,
 	NULL,
 };