runtime: fix deadlock in network poller The invariant is that there must be at least one running P or a thread polling network. It was broken. Fixes issue 5216.
runtime: fix deadlock in network poller
The invariant is that there must be at least one running P or a thread polling network.
It was broken.
Fixes issue 5216.

R=golang-dev, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/8459043

tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme