xfs: read and write metadata inode directory tree

xfs: read and write metadata inode directory tree

Source kernel commit e0f091c40bf1287a122dcddd8aa4e6ad06ce441f

Plumb in the bits we need to load metadata inodes from a named entry in
a metadir directory, create (or hardlink) inodes into a metadir
directory, create metadir directories, and flag inodes as being metadata
files.

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