xfs: bail out of scrub immediately if scan incomplete

If a scrubber cannot complete its check and signals an incomplete check,
we must bail out immediately without updating health status, trying a
repair, etc. because our scan is incomplete and we therefore do not know
much more.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>