Checksum errors may not be counted

[don-brady]

Motivation and Context

Fix regression seen in issue #11545 where checksum errors where not being counted or showing up in a zpool event.

Description

1. Added a zfs_ereport_clear() function that will clear recent events after a pool scrub or vdev clear.
2. Removed condition for increments of vd->vdev_stat.vs_checksum_errors (i.e. always increment).

How Has This Been Tested?

1. Added new ZTS test, zpool_events_clear_retained, that verifies that after a scrub or vdev clear that new errors to the same block generate an event and are counted.
2. ZFS Test suite ran against zpool_events and events tags
3. Stock ztest run

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.