context: use atomic operation in ctx.Err

context: use atomic operation in ctx.Err

oos: darwin
goarch: arm64
pkg: context
cpu: Apple M1 Pro
               │ /tmp/bench.0.mac │          /tmp/bench.1.mac           │
               │      sec/op      │   sec/op     vs base                │
ErrOK-10             13.750n ± 1%   2.080n ± 0%  -84.87% (p=0.000 n=10)
ErrCanceled-10       13.530n ± 1%   3.248n ± 1%  -76.00% (p=0.000 n=10)
geomean               13.64n        2.599n       -80.94%

goos: linux
goarch: amd64
pkg: context
cpu: Intel(R) Xeon(R) CPU @ 2.30GHz
               │ /tmp/bench.0.linux │         /tmp/bench.1.linux          │
               │       sec/op       │   sec/op     vs base                │
ErrOK-16               21.435n ± 0%   4.243n ± 0%  -80.21% (p=0.000 n=10)
ErrCanceled-16         21.445n ± 0%   5.070n ± 0%  -76.36% (p=0.000 n=10)
geomean                 21.44n        4.638n       -78.37%

Fixes #72040

Change-Id: I3b337ab1934689d2da4134492ee7c5aac8f92845
Reviewed-on: https://go-review.googlesource.com/c/go/+/653795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>