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

scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,d0f/go-zh,scirelli/scirelli-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,d0f/go-zh,mhennings/marcohennings-go,webfd/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,sanjosh/sanjos100-tipc