xfsprogs: shard the realtime section

Right now, the realtime section uses a single pair of metadata inodes to
store the free space information.  This presents a scalability problem
since every thread trying to allocate or free rt extents have to lock
these files.  It would be very useful if we could begin to tackle these
problems by sharding the realtime section, so create the notion of
realtime groups, which are similar to allocation groups on the data
section.

While we're at it, define a superblock to be stamped into the start of
each rt section.  This enables utilities such as blkid to identify block
devices containing realtime sections, and helpfully avoids the situation
where a file extent can cross an rtgroup boundary.

The best advantage for rtgroups will become evident later when we get to
adding rmap and reflink to the realtime volume, since the geometry
constraints are the same for rt groups and AGs.  Hence we can reuse all
that code directly.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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