If there are no netpoll waiters then calling netpoll will never find any
goroutines. The later blocking netpoll in findrunnable already has this
optimization.

With golang.org/cl/78538 also applied, this change has a small impact on
latency:

name                             old time/op  new time/op  delta
WakeupParallelSpinning/0s-12     13.6µs ± 1%  13.7µs ± 1%    ~     (p=0.873 n=19+20)
WakeupParallelSpinning/1µs-12    17.7µs ± 0%  17.6µs ± 0%  -0.31%  (p=0.000 n=20+20)
WakeupParallelSpinning/2µs-12    20.2µs ± 2%  19.9µs ± 1%  -1.59%  (p=0.000 n=20+19)
WakeupParallelSpinning/5µs-12    32.0µs ± 1%  32.1µs ± 1%    ~     (p=0.201 n=20+19)
WakeupParallelSpinning/10µs-12   51.7µs ± 0%  51.4µs ± 1%  -0.60%  (p=0.000 n=20+18)
WakeupParallelSpinning/20µs-12   92.2µs ± 0%  92.2µs ± 0%    ~     (p=0.474 n=19+19)
WakeupParallelSpinning/50µs-12    215µs ± 0%   215µs ± 0%    ~     (p=0.319 n=20+19)
WakeupParallelSpinning/100µs-12   330µs ± 2%   331µs ± 2%    ~     (p=0.296 n=20+19)
WakeupParallelSyscall/0s-12       127µs ± 0%   126µs ± 0%  -0.57%  (p=0.000 n=18+18)
WakeupParallelSyscall/1µs-12      129µs ± 0%   128µs ± 1%  -0.43%  (p=0.000 n=18+19)
WakeupParallelSyscall/2µs-12      131µs ± 1%   130µs ± 1%  -0.78%  (p=0.000 n=20+19)
WakeupParallelSyscall/5µs-12      137µs ± 1%   136µs ± 0%  -0.54%  (p=0.000 n=18+19)
WakeupParallelSyscall/10µs-12     147µs ± 1%   146µs ± 0%  -0.58%  (p=0.000 n=18+19)
WakeupParallelSyscall/20µs-12     168µs ± 0%   167µs ± 0%  -0.52%  (p=0.000 n=19+19)
WakeupParallelSyscall/50µs-12     228µs ± 0%   227µs ± 0%  -0.37%  (p=0.000 n=19+18)
WakeupParallelSyscall/100µs-12    329µs ± 0%   328µs ± 0%  -0.28%  (p=0.000 n=20+18)

There is a bigger improvement in CPU utilization. Before this CL, these
benchmarks spent 12% of cycles in netpoll, which are gone after this CL.

This also fixes the sched.lastpoll load, which should be atomic.

Change-Id: I600961460608bd5ba3eeddc599493d2be62064c6
Reviewed-on: https://go-review.googlesource.com/78915
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Austin Clements <austin@google.com>