xfs: delete duplicated tp->t_dqinfo null check and allocation

The function xfs_trans_mod_dquot_byino() wraps around
xfs_trans_mod_dquot() to account for quotas, and also there is the
function call chain xfs_trans_reserve_quota_bydquots -> xfs_trans_dqresv
-> xfs_trans_mod_dquot, both of them do the duplicated null check and
allocation. Thus we can delete the duplicated operation from them.

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