unix: implement Linux AF_XDP sockets

Provides access to the Linux AF_XDP socket family.  For more
information on AF_XDP, see:
https://www.kernel.org/doc/html/latest/networking/af_xdp.html.

Change-Id: I7efb82c4a67da3e15932e3c0d882f0bad7ba16ef
Reviewed-on: https://go-review.googlesource.com/136695
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux.go

@@ -692,6 +692,24 @@ func (sa *SockaddrVM) sockaddr() (unsafe.Pointer, _Socklen, error) {
 	return unsafe.Pointer(&sa.raw), SizeofSockaddrVM, nil
 }
 
+type SockaddrXDP struct {
+	Flags        uint16
+	Ifindex      uint32
+	QueueID      uint32
+	SharedUmemFD uint32
+	raw          RawSockaddrXDP
+}
+
+func (sa *SockaddrXDP) sockaddr() (unsafe.Pointer, _Socklen, error) {
+	sa.raw.Family = AF_XDP
+	sa.raw.Flags = sa.Flags
+	sa.raw.Ifindex = sa.Ifindex
+	sa.raw.Queue_id = sa.QueueID
+	sa.raw.Shared_umem_fd = sa.SharedUmemFD
+
+	return unsafe.Pointer(&sa.raw), SizeofSockaddrXDP, nil
+}
+
 func anyToSockaddr(fd int, rsa *RawSockaddrAny) (Sockaddr, error) {
 	switch rsa.Addr.Family {
 	case AF_NETLINK:
@@ -793,6 +811,15 @@ func anyToSockaddr(fd int, rsa *RawSockaddrAny) (Sockaddr, error) {
 			}
 			return sa, nil
 		}
+	case AF_XDP:
+		pp := (*RawSockaddrXDP)(unsafe.Pointer(rsa))
+		sa := &SockaddrXDP{
+			Flags:        pp.Flags,
+			Ifindex:      pp.Ifindex,
+			QueueID:      pp.Queue_id,
+			SharedUmemFD: pp.Shared_umem_fd,
+		}
+		return sa, nil
 	}
 	return nil, EAFNOSUPPORT
 }