Fix the ZFS checksum error histograms with larger record sizes

My analysis in PR #14716 was incorrect. Each histogram bucket contains
the number of incorrect bits, by position in a 64-bit word, over the
entire record. 8-bit buckets can overflow for record sizes above 2k.
To forestall that, saturate each bucket at 255. That should still get
the point across: either all bits are equally wrong, or just a couple
are.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alan Somers <asomers@gmail.com>
Sponsored-by: Axcient
Closes #15049