zed: Take no action on scrub/resilver checksum errors

When scrubbing/resilvering a pool it can be counter productive to
cancel the scan and kick of a replace operation to a hot spare
when encountering checksum errors. In this case, the best course
of action is to allow the scrub/resilver to complete as quickly
as possible and to keep the vdevs fully online if possible.

Realistically, this is less of an issue for a RAIDZ since a
traditional resilver must be used and checksums will be verified.
However, this is not the case for a mirror or dRAID pool which is
sequentially resilvered and checksum verification is deferred
until after the replace operation completes.

Regardless, we apply this policy to all pool types since it's
a good idea for all vdevs. Degrading additional vdevs has the
potential to make a bad situation worse. Note the checksum
errors will still be reported as both an event and by
zpool status. This change only prevents the ZED from
proactively taking any action.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13499