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