common/fuzzy: fix some problems with the online-then-offline repair strategy

common/fuzzy: fix some problems with the online-then-offline repair strategy

While auditing the fuzz tester code, I noticed there were numerous
problems with the online-then-offline repair strategy -- the stages of
the strategy are not consistently logged to the kernel log, some of the
error messages don't identify /which/ scrubber we're calling, we don't
do a pre-repair check to make sure we detect the fuzzed fields, and we
don't actually re-run online scrub after a repair to make sure that it's
ok.  Disable xfs_repair prefetch to reduce the possibility of OOM kills.
Rework the error messages to make reading the golden output easier.

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