hash/maphash: manually inline setSeed

Provides a minor performance win.

name            old time/op    new time/op    delta
Hash8Bytes-8      16.5ns ± 2%    16.5ns ± 4%    ~     (p=0.407 n=27+29)
Hash320Bytes-8    58.5ns ± 2%    55.0ns ± 2%  -6.01%  (p=0.000 n=29+28)
Hash1K-8           195ns ± 1%     190ns ± 2%  -2.23%  (p=0.000 n=30+30)
Hash8K-8          1.59µs ± 2%    1.57µs ± 2%  -0.88%  (p=0.002 n=30+30)

name            old speed      new speed      delta
Hash8Bytes-8     484MB/s ± 2%   485MB/s ± 4%    ~     (p=0.417 n=27+29)
Hash320Bytes-8  5.47GB/s ± 2%  5.82GB/s ± 2%  +6.39%  (p=0.000 n=29+28)
Hash1K-8        5.26GB/s ± 1%  5.39GB/s ± 2%  +2.29%  (p=0.000 n=30+30)
Hash8K-8        5.16GB/s ± 2%  5.21GB/s ± 2%  +0.89%  (p=0.002 n=30+30)

Updates #42710

Change-Id: Ia0d7264b648f96099202de21c6b69a9c1776f6c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/278759


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>