if_wg: fix access to noise_local->l_has_identity and l_private

These members are protected by the identity lock, so rlock it in
noise_remote_alloc() and then assert that we have it held to some extent
in noise_precompute_ss().

PR: 276392

(cherry picked from commit 7a4d1d1df0b2e369adcb32aea9ef8c180f885751)