xfs: add realtime refcount btree inode to metadata directory

xfs: add realtime refcount btree inode to metadata directory

Add a metadir path to select the realtime refcount btree inode and load
it at mount time.  The rtrefcountbt inode will have a unique extent format
code, which means that we also have to update the inode validation and
flush routines to look for it.

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