runtime: only call netpoll if netpollinited returns true
runtime: only call netpoll if netpollinited returns true

This fixes a race on old Linux kernels, in which we might temporarily
set epfd to an invalid value other than -1. It's also the right thing
to do. No test because the problem only occurs on old kernels.

Fixes #22606

Change-Id: Id84bdd6ae6d7c5d47c39e97b74da27576cb51a54
Reviewed-on: https://go-review.googlesource.com/76319
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

CAFxX/go,josharian/go,Cofyc/go,akutz/go,Cofyc/go,akutz/go,CAFxX/go,josharian/go,CAFxX/go,Cofyc/go,Cofyc/go,Cofyc/go,CAFxX/go,Cofyc/go,CAFxX/go,akutz/go,codestation/go,akutz/go,golang/go,CAFxX/go,josharian/go,josharian/go,golang/go,golang/go,Cofyc/go,josharian/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,josharian/go,golang/go,cloudflare/go,akutz/go,codestation/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,codestation/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,codestation/go,akutz/go,akutz/go,golang/go,golang/go,nawawi/go,josharian/go,codestation/go,CAFxX/go,codestation/go,nawawi/go,nawawi/go,cloudflare/go,codestation/go,codestation/go,Cofyc/go,golang/go,josharian/go,christopher-henderson/Go