runtime: skip netpoll check if there are no waiters

runtime: skip netpoll check if there are no waiters

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>