hash/maphash: increase the buffer size

hash/maphash: increase the buffer size

This helps a lot for larger writes.

name            old time/op    new time/op    delta
Hash8Bytes-8      16.5ns ± 4%    16.8ns ± 2%   +1.76%  (p=0.000 n=29+30)
Hash320Bytes-8    55.0ns ± 2%    41.4ns ± 2%  -24.64%  (p=0.000 n=28+28)
Hash1K-8           190ns ± 2%     130ns ± 3%  -31.65%  (p=0.000 n=30+30)
Hash8K-8          1.57µs ± 2%    1.05µs ± 3%  -33.01%  (p=0.000 n=30+29)

name            old speed      new speed      delta
Hash8Bytes-8     485MB/s ± 4%   476MB/s ± 2%   -1.73%  (p=0.000 n=29+30)
Hash320Bytes-8  5.82GB/s ± 2%  7.72GB/s ± 3%  +32.55%  (p=0.000 n=28+29)
Hash1K-8        5.39GB/s ± 2%  7.88GB/s ± 3%  +46.32%  (p=0.000 n=30+30)
Hash8K-8        5.21GB/s ± 2%  7.77GB/s ± 3%  +49.28%  (p=0.000 n=30+29)

Updates #42710

Change-Id: Idaf4b2a8a41fc62fc16b54c9358cf2cc7009cf29
Reviewed-on: https://go-review.googlesource.com/c/go/+/278760
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>