net: provide __sys_shutdown_sock() that takes a socket

No functional changes in this patch, needed to provide io_uring support
for shutdown(2).

Cc: netdev@vger.kernel.org
Cc: David S. Miller <davem@davemloft.net>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>