When acquiring from a semaphore could proceed without contention, the
previous behavior was to always do so, even when the provided context
was done. This was the documented behavior, but it could lead to
confusion. It isn't much more expensive to check the context error, so
cancel acquisition if it's done.

Fixes golang/go#63615.

goos: linux
goarch: amd64
pkg: golang.org/x/sync/semaphore
cpu: 12th Gen Intel(R) Core(TM) i5-1235U
                                        │  old.bench  │             new.bench              │
                                        │   sec/op    │   sec/op     vs base               │
AcquireSeq/Weighted-acquire-1-1-1-12      26.45n ± 2%   27.25n ± 3%  +3.04% (p=0.001 n=20)
AcquireSeq/Weighted-acquire-2-1-1-12      26.96n ± 1%   27.12n ± 1%       ~ (p=0.104 n=20)
AcquireSeq/Weighted-acquire-16-1-1-12     26.07n ± 3%   27.48n ± 1%  +5.45% (p=0.000 n=20)
AcquireSeq/Weighted-acquire-128-1-1-12    26.19n ± 2%   27.24n ± 1%  +4.01% (p=0.000 n=20)
AcquireSeq/Weighted-acquire-2-2-1-12      25.61n ± 1%   25.99n ± 2%       ~ (p=0.066 n=20)
AcquireSeq/Weighted-acquire-16-2-8-12     209.6n ± 2%   211.0n ± 3%       ~ (p=0.280 n=20)
AcquireSeq/Weighted-acquire-128-2-64-12   1.669µ ± 1%   1.721µ ± 2%  +3.09% (p=0.000 n=20)
AcquireSeq/Weighted-acquire-2-1-2-12      51.08n ± 1%   53.03n ± 2%  +3.82% (p=0.000 n=20)
AcquireSeq/Weighted-acquire-16-8-2-12     52.48n ± 2%   53.66n ± 2%  +2.26% (p=0.028 n=20)
AcquireSeq/Weighted-acquire-128-64-2-12   52.27n ± 1%   53.71n ± 2%  +2.75% (p=0.000 n=20)
geomean                                   60.06n        61.69n       +2.71%

Change-Id: I0ae1a0bb6c027461ac1a9ee71c51efd8427ab308
Reviewed-on: https://go-review.googlesource.com/c/sync/+/536275
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>