net: fill in MODULE_DESCRIPTION() for AF_PACKET

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add description to net/packet/af_packet.c

Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240104144119.1319055-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>