Trim needless zeroes from checksum events

Trim needless zeroes from checksum events

The ereport.fs.zfs.checksum event contains histograms of the bits that
were wrongly set or cleared according to their bit position in a 64-bit
word.  So the maximum value that any histogram bucket could have would
be 64.  But ZFS currently uses a uint32_t to hold each bucket.  As a
result, the event report is full of needless zeroes.

Change the bucket size to uint8_t, stripping 768 needless zeros from
each event.

Original event format:
```
 class=ereport.fs.zfs.checksum ena=639460469834258433 pool=testpool.1933 pool_guid=4979719877084416563 pool_state=0 pool_context=0 pool_failmode=wait vdev_guid=4136721804819128578 vdev_type=file vdev_path=/tmp/kyua.1TxP3A/2/work/file1.1933 vdev_ashift=9 vdev_complete_ts=609837019678 vdev_delta_ts=33450 vdev_read_errors=0 vdev_write_errors=0 vdev_cksum_errors=20 vdev_delays=0 parent_guid=2751977006639883417 parent_type=raidz vdev_spare_guids= zio_err=0 zio_flags=1048752 zio_stage=4194304 zio_pipeline=65011712 zio_delay=0 zio_timestamp=0 zio_delta=0 zio_priority=4 zio_offset=702976 zio_size=1024 zio_objset=24 zio_object=0 zio_level=3 zio_blkid=0 bad_ranges=0000000000000400 bad_ranges_min_gap=8 bad_range_sets=0000079e bad_range_clears=00000854 bad_set_histogram=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 bad_cleared_histogram=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 time=00000016806457270000000323406839 eid=458
```

New format:
```
 class=ereport.fs.zfs.checksum ena=96599319807790081 pool=testpool.1933 pool_guid=1236902063710799041 pool_state=0 pool_context=0 pool_failmode=wait vdev_guid=2774253874431514999 vdev_type=file vdev_path=/tmp/kyua.6Temlq/2/work/file1.1933 vdev_ashift=9 vdev_complete_ts=92124283803 vdev_delta_ts=46670 vdev_read_errors=0 vdev_write_errors=0 vdev_cksum_errors=20 vdev_delays=0 parent_guid=8090931855087882905 parent_type=raidz vdev_spare_guids= zio_err=0 zio_flags=1048752 zio_stage=4194304 zio_pipeline=65011712 zio_delay=0 zio_timestamp=0 zio_delta=0 zio_priority=4 zio_offset=1028608 zio_size=512 zio_objset=0 zio_object=0 zio_level=0 zio_blkid=4 bad_ranges=0000000000000200 bad_ranges_min_gap=8 bad_range_sets=0000061f bad_range_clears=000001f4 bad_set_histogram=1719161c1c1c101618171a151a1a19161e1c171d1816161c191f1a18192117191c131d171b1613151a171419161a1b1319101b14171b18151e191a1b141a1c17 bad_cleared_histogram=06090a0808070a0b020609060506090a01090a050a0a0509070609080d050d0607080d060507080c04070807070a0608020c080c080908040808090a05090a07 time=00000016806477050000000604157480 eid=62
```

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Alan Somers <asomers@FreeBSD.org>
Sponsored-by: Axcient
Closes #14716