fs/xfs: fix f_ffree value for statfs when project quota is set

When project is set, we should use inode limit minus the used count

Signed-off-by: Ye Yin <dbyin@tencent.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>