internal/netlink: optional recvmsg without netpoll

For draining the socket receive buffer.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/netlink/netlink.go

@@ -135,6 +135,23 @@ func (c *Conn) Recvmsg(
 
 	var n int
 	data = c.buf[:]
+
+	if ctx == nil {
+		rcErr := c.raw.Control(func(fd uintptr) {
+			n, _, recvflags, from, err = syscall.Recvmsg(int(fd), data, nil, flags)
+		})
+		if n >= 0 {
+			data = data[:n]
+		}
+
+		if err != nil {
+			err = os.NewSyscallError("recvmsg", err)
+		} else {
+			err = rcErr
+		}
+		return
+	}
+
 	done := make(chan error, 1)
 	go func() {
 		rcErr := c.raw.Read(func(fd uintptr) (done bool) {