net: close correct file descriptor when netpoll registration fails

In (*netFD).accept, if initializing the *netFD associated with the
new connection fails, the listen FD is closed, rather than the FD
associated with the new connection. Close the correct FD instead.

Fixes #34392

Change-Id: I7bf3469d661e6d30cbd4b12f5f5fd330a81a541b
Reviewed-on: https://go-review.googlesource.com/c/go/+/196778
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/fd_unix.go

@@ -248,7 +248,7 @@ func (fd *netFD) accept() (netfd *netFD, err error) {
 		return nil, err
 	}
 	if err = netfd.init(); err != nil {
-		fd.Close()
+		netfd.Close()
 		return nil, err
 	}
 	lsa, _ := syscall.Getsockname(netfd.pfd.Sysfd)