xfs: support file data forks containing metadata btrees

xfs: support file data forks containing metadata btrees

Create a new fork format type for metadata btrees.  This fork type
requires that the inode is in the metadata directory tree, and only
applies to the data fork.  The actual type of the metadata btree itself
is determined by the di_metatype field.

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