xfs: report realtime block quota limits on realtime directories

On the data device, calling statvfs on a projinherit directory results
in the block and avail counts being curtailed to the project quota block
limits, if any are set.  Do the same for realtime files or directories,
only use the project quota rt block limits.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>