xfs: quota limit statvfs available blocks

A "df" run on an NFS client of an exported XFS file system reports
the wrong information for "available" blocks.  When a block quota is
enforced, the amount reported as free is limited by the quota, but
the amount reported available is not (and should be).

Reported-by: Guk-Bong, Kwon <gbkwon@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>