net: do not use RLock around Accept

It might be non-blocking, but it also might be blocking.
Cannot take the chance, as Accept might block indefinitely
and make it impossible to acquire ForkLock exclusively
(during fork+exec).

Fixes issue 4737.

R=golang-dev, dave, iant, mikioh.mikioh
CC=golang-dev
https://codereview.appspot.com/7309050