Fix the ZFS checksum error histograms with larger record sizes

My analysis in PR openzfs#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 <[email protected]>
Signed-off-by: Alan Somers <[email protected]>
Sponsored-by: Axcient
Closes openzfs#15049

module/zfs/zfs_fm.c

@@ -790,7 +790,7 @@ update_histogram(uint64_t value_arg, uint8_t *hist, uint32_t *count)
 	/* We store the bits in big-endian (largest-first) order */
 	for (i = 0; i < 64; i++) {
 		if (value & (1ull << i)) {
-			hist[63 - i]++;
+			hist[63 - i] = MAX(hist[63 - i], hist[63 - i] + 1);
 			++bits;
 		}
 	}