net: fix data race on fd.sysfd

Fixes #4369.

Remove the check for fd.sysfd < 0, the first line of fd.accept() tests if the fd is open correctly and will handle the fd being closed during accept.

R=dvyukov, bradfitz
CC=golang-dev
https://golang.org/cl/6843076

src/pkg/net/tcpsock_posix.go

@@ -231,7 +231,7 @@ type TCPListener struct {
 // AcceptTCP accepts the next incoming call and returns the new connection
 // and the remote address.
 func (l *TCPListener) AcceptTCP() (c *TCPConn, err error) {
-	if l == nil || l.fd == nil || l.fd.sysfd < 0 {
+	if l == nil || l.fd == nil {
 		return nil, syscall.EINVAL
 	}
 	fd, err := l.fd.accept(sockaddrToTCP)