network: speed up createSocket() by giving protocal 0 in socket()

In practice, we found that createSocket() could cost up to 40ms,
which is an unacceptable overhead in FaaS(Function as a Service)
scenario. The root cause is the bind() syscall runs into the slow
path in kernel and invokes synchronize_net(), which is an expensive
call.

From kernel commit[1] we learned that we could bypass this slow
path by adjusting the socket() syscall before we call bind(). By
giving the empty protocal number, the packet_create() will not
call __register_prot_hook(), in which the kernel turn on the flag
packet_sock.running. So when it comes to packet_do_bind(), it
won't call synchronize_net() for there is no prot_hook to unregister.

Worst cost for bind() tested on Intel Xeon E5-2682 with Linux
kernel 5.4 over 100 times:

  * before: 39633.274 us
  * after:     25.098 us

Reference:
[1] https://github.com/torvalds/linux/commit/902fefb82ef72a50c78cb4a20cc954b037a98d1c

Signed-off-by: Tianyu Zhou <albert.zty@antgroup.com>

runsc/sandbox/network.go

@@ -360,7 +360,7 @@ type socketEntry struct {
 func createSocket(iface net.Interface, ifaceLink netlink.Link, enableGSO bool) (*socketEntry, error) {
 	// Create the socket.
 	const protocol = 0x0300 // htons(ETH_P_ALL)
-	fd, err := unix.Socket(unix.AF_PACKET, unix.SOCK_RAW, protocol)
+	fd, err := unix.Socket(unix.AF_PACKET, unix.SOCK_RAW, 0) // pass protocol 0 to avoid slow bind()
 	if err != nil {
 		return nil, fmt.Errorf("unable to create raw socket: %v", err)
 	}