crypto/hmac: panic if reusing hash.Hash values

Also put Reset in the correct place for the other
benchmarks.

name           old time/op    new time/op    delta
NewWriteSum-8    1.01µs ± 0%    1.01µs ± 1%   ~     (p=0.945 n=9+9)

name           old speed      new speed      delta
NewWriteSum-8  31.7MB/s ± 0%  31.6MB/s ± 1%   ~     (p=0.948 n=9+9)

name           old alloc/op   new alloc/op   delta
NewWriteSum-8      544B ± 0%      544B ± 0%   ~     (all equal)

name           old allocs/op  new allocs/op  delta
NewWriteSum-8      7.00 ± 0%      7.00 ± 0%   ~     (all equal)

Fixes #41089

Change-Id: I3dae660adbe4993963130bf3c2636bd53899164b
Reviewed-on: https://go-review.googlesource.com/c/go/+/261960


Trust: Katie Hockman <katie@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>