fuzzy: rework scrub stress output filtering

fuzzy: rework scrub stress output filtering

Rework the output filtering functions for scrub stress tests: first, we
should use _filter_scratch to avoid leaking the scratch fs details to
the output.  Second, for scrub and repair, change the filter elements to
reflect outputs that don't indicate failure (such as busy resources,
preening requests, and insufficient space to do anything).  Finally,
change the _require function to check that filter functions have been
sourced.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>