xfs: define the on-disk format for the metadir feature

xfs: define the on-disk format for the metadir feature

Source kernel commit: 4f3d4dd1b04b2ba0bf236fbaa3c3c0c669aa5a47

Define the on-disk layout and feature flags for the metadata inode
directory feature.  Add a xfs_sb_version_hasmetadir for benefit of
xfs_repair, which needs to know where the new end of the superblock
lies.

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