xfs: don't fill statvfs with project quota for a directory if it was not enabled.

Check if the project quota is running or not before performing
xfs_qm_statvfs(), just return if not.  Otherwise the ASSERT
XFS_IS_QUOTA_RUNNING in xfs_qm_dqget will be popped.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Ben Myers <bpm@sgi.com>