xfs: skip fscounters comparisons when the scan is incomplete

If any part of the per-AG summary counter scan loop aborts without
collecting all of the data we need, the scrubber's observation data will
be invalid.  Set the incomplete flag so that we abort the scrub without
reporting false corruptions.  Document the data dependency here too.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>