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

module/zfs/zfs_fm.c

@@ -755,8 +755,8 @@ zfs_ereport_start(nvlist_t **ereport_out, nvlist_t **detector_out,
 
 typedef struct zfs_ecksum_info {
 	/* histograms of set and cleared bits by bit number in a 64-bit word */
-	uint32_t zei_histogram_set[sizeof (uint64_t) * NBBY];
-	uint32_t zei_histogram_cleared[sizeof (uint64_t) * NBBY];
+	uint8_t zei_histogram_set[sizeof (uint64_t) * NBBY];
+	uint8_t zei_histogram_cleared[sizeof (uint64_t) * NBBY];
 
 	/* inline arrays of bits set and cleared. */
 	uint64_t zei_bits_set[ZFM_MAX_INLINE];
@@ -781,7 +781,7 @@ typedef struct zfs_ecksum_info {
 } zfs_ecksum_info_t;
 
 static void
-update_histogram(uint64_t value_arg, uint32_t *hist, uint32_t *count)
+update_histogram(uint64_t value_arg, uint8_t *hist, uint32_t *count)
 {
 	size_t i;
 	size_t bits = 0;
@@ -1052,10 +1052,10 @@ annotate_ecksum(nvlist_t *ereport, zio_bad_cksum_t *info,
 	} else {
 		fm_payload_set(ereport,
 		    FM_EREPORT_PAYLOAD_ZFS_BAD_SET_HISTOGRAM,
-		    DATA_TYPE_UINT32_ARRAY,
+		    DATA_TYPE_UINT8_ARRAY,
 		    NBBY * sizeof (uint64_t), eip->zei_histogram_set,
 		    FM_EREPORT_PAYLOAD_ZFS_BAD_CLEARED_HISTOGRAM,
-		    DATA_TYPE_UINT32_ARRAY,
+		    DATA_TYPE_UINT8_ARRAY,
 		    NBBY * sizeof (uint64_t), eip->zei_histogram_cleared,
 		    NULL);
 	}