xfs: directly return if the delta equal to zero

xfs: directly return if the delta equal to zero

The xfs_trans_mod_dquot() function will allocate new tp->t_dqinfo if
it is NULL and make the changes in the tp->t_dqinfo->dqs[XFS_QM_TRANS
_{USR,GRP,PRJ}]. Nowadays seems none of the callers want to join the
dquots to the transaction and push them to device when the delta is
zero. Actually, most of time the caller would check the delta and go
on only when the delta value is not zero, so we should bail out when
it is zero.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>