target: Obtain se_node_acl->acl_kref during get_initiator_node_acl

This patch addresses a long standing race where obtaining
se_node_acl->acl_kref in __transport_register_session()
happens a bit too late, and leaves open the potential
for core_tpg_del_initiator_node_acl() to hit a NULL
pointer dereference.

Instead, take ->acl_kref in core_tpg_get_initiator_node_acl()
while se_portal_group->acl_node_mutex is held, and move the
final target_put_nacl() from transport_deregister_session()
into transport_free_session() so that fabric driver login
failure handling using the modern method to still work
as expected.

Also, update core_tpg_get_initiator_node_acl() to take
an extra reference for dynamically generated acls for
demo-mode, before returning to fabric caller.  Also
update iscsi-target sendtargets special case handling
to use target_tpg_has_node_acl() when checking if
demo_mode_discovery == true during discovery lookup.

Note the existing wait_for_completion(&acl->acl_free_comp)
in core_tpg_del_initiator_node_acl() does not change.

Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Andy Grover <agrover@redhat.com>
Cc: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -3435,7 +3435,7 @@ iscsit_build_sendtargets_response(struct iscsi_cmd *cmd,
 
 			if ((tpg->tpg_attrib.generate_node_acls == 0) &&
 			    (tpg->tpg_attrib.demo_mode_discovery == 0) &&
-			    (!core_tpg_get_initiator_node_acl(&tpg->tpg_se_tpg,
+			    (!target_tpg_has_node_acl(&tpg->tpg_se_tpg,
 				cmd->conn->sess->sess_ops->InitiatorName))) {
 				continue;
 			}