wg: detach bpf upon destroy as well

bpfattach() is called in wg_clone_create(), but the bpfdetach() is
missing from wg_close_destroy(). Add the missing bpfdetach() to avoid
leaking both the associated bpf bits as well as the ifnet that bpf will
hold a reference to.

PR: 276526

(cherry picked from commit 43be2d7aaf25b719aec8f49aab110c0061f1edec)