xfs: define the format of rt groups

xfs: define the format of rt groups

Source kernel commit: f13677b8a76a61cbb1c937d250a3bd45019748e8

Define the ondisk format of realtime group metadata, and a superblock
for realtime volumes.  rt supers are protected by a separate rocompat
bit so that we can leave them off if the rt device is zoned.

Add a xfs_sb_version_hasrtgroups so that xfs_repair knows how to zero
the tail of superblocks.

For rt group enabled file systems there is a separate bitmap and summary
file for each group and thus the number of bitmap and summary blocks
needs to be calculated differently.

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