if_wg: Missing radix unlock can cause deadlock

In function 'wg_aip_add()', the error path of returning ENOMEM when
(node == NULL) is forgetting to unlock the radix tree, and thus may lead
to a deadlock.

PR: 275001
Reviewed by: kp
MFC after: 1 week

(cherry picked from commit dcc4d2939f789a6d1f272ffeab2068ba2b7525ea)