xfs: fix return code when fatal signal encountered during dquot scrub

If the scrub process is sent a fatal signal while we're checking dquots,
the predicate for this will set the error code to -EINTR.  Don't then
squash that into -ECANCELED, because the wrong errno turns up in the
trace output.

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