xfs: clean up error handling in xfs_trans_dqresv

Move the error code selection after the goto label and fold the
xfs_quota_error helper into it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Alex Elder <aelder@sgi.com>