net/packet: Add getsockopt support for PACKET_COPY_THRESH

Currently getsockopt does not support PACKET_COPY_THRESH,
and we are unable to get the value of PACKET_COPY_THRESH
socket option through getsockopt.

This patch adds getsockopt support for PACKET_COPY_THRESH.

In addition, this patch converts access to copy_thresh to
READ_ONCE/WRITE_ONCE.

Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://lore.kernel.org/r/AM6PR03MB58487A9704FD150CF76F542899272@AM6PR03MB5848.eurprd03.prod.outlook.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>