xfs: fix dquot scrub loop cancellation

When xchk_quota_item figures out that it needs to terminate the scrub
operation, it needs to return some error code to abort the loop, but
instead it returns zero and the loop keeps running.  Fix this by making
it use ECANCELED, and fix the other loop bailout condition check at the
bottom too.

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