wg: Fix noise_remote_alloc() to acquire 'l_identity_lock' lock

wg: Fix noise_remote_alloc() to acquire 'l_identity_lock' lock

The 'l_identity_lock' lock must be acquired to access 'l_has_identity'
and 'l_private' members; i.e., noise_precompute_ss() must be called with
the 'l_identity_lock' locked.  So fix noise_remote_alloc() to acquire
the lock before calling noise_precompute_ss().  Meanwhile, add an
assertion to the latter to assert the required lock is held.