xfs_repair: mark space used by metadata files

xfs_repair: mark space used by metadata files

Track space used by metadata files as a separate incore extent type.
This ensures that we can warn about cross-linked metadata files, even
though we are going to rebuild the entire metadata directory tree in the
end.

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