metadump: Define metadump v2 ondisk format structures and macros

metadump: Define metadump v2 ondisk format structures and macros

The corresponding metadump file's disk layout is as shown below,

     |------------------------------|
     | struct xfs_metadump_header   |
     |------------------------------|
     | struct xfs_meta_extent 0     |
     | Extent 0's data              |
     | struct xfs_meta_extent 1     |
     | Extent 1's data              |
     | ...                          |
     | struct xfs_meta_extent (n-1) |
     | Extent (n-1)'s data          |
     |------------------------------|

The "struct xfs_metadump_header" is followed by alternating series of "struct
xfs_meta_extent" and the extent itself.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>