net: Remove an unmatched unlock of ForkLock

Remove an RUnlock of syscall.ForkLock with no matching RLock.
Holding ForkLock in netFD.dup is unnecessary: dupCloseOnExecOld
locks and unlocks the lock on its own and dupCloseOnExec doesn't
need the ForkLock to be held.

LGTM=iant
R=golang-codereviews, bradfitz, iant
CC=golang-codereviews
https://golang.org/cl/99800044

src/pkg/net/fd_unix.go

@@ -482,7 +482,6 @@ func dupCloseOnExecOld(fd int) (newfd int, err error) {
 func (fd *netFD) dup() (f *os.File, err error) {
 	ns, err := dupCloseOnExec(fd.sysfd)
 	if err != nil {
-		syscall.ForkLock.RUnlock()
 		return nil, &OpError{"dup", fd.net, fd.laddr, err}
 	}