phonet: take correct lock to peek at the RX queue

The receive queue is protected by its embedded spin-lock, not the
socket lock, so we need the former lock here (and only that one).

Fixes: 107d0d9b ("Phonet: Phonet datagram transport protocol")
Reported-by: Luosili <rootlab@huawei.com>
Signed-off-by: Rémi Denis-Courmont <courmisch@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240218081214.4806-1-remi@remlab.net


Signed-off-by: Paolo Abeni <pabeni@redhat.com>