The cycle is *netFD -> cw chanl *netFD in struct ->
same *netFD in channel read buffer.

Because channels are finalized, the cycle makes them
uncollectable.  A better fix is to make channels not
finalized anymore, and that will happen, but this is
an easy, reasonable workaround until then.

Another good fix would be to zero the channel receive
buffer entry after the receive.  That too will happen.

R=r
CC=golang-dev
https://golang.org/cl/875043