xfs: return maximum free size from xfs_rtany_summary()

Instead of only returning whether there is any free space, return the
maximum size, which is fast thanks to the previous commit. This will be
used by two upcoming optimizations.

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