xfs: simplify the flags setting in xfs_qm_scall_quotaon

Simplify the setting of the flags value, and only consider
quota enforcement stuff here.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>