Scrubbing directories, quotas, and fs counters all involve iterating
some collection of metadata items.  The per-item scrub functions for
these three are missing some of the components they need to be able to
check for a fatal signal and terminate early.

Per-item scrub functions need to call xchk_should_terminate to look for
fatal signals, and they need to check the scrub context's corruption
flag because there's no point in continuing a scan once we've decided
the data structure is bad.  Add both of these where missing.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>