xfs: add a helper to prevent bmap merges across rtgroup boundaries

xfs: add a helper to prevent bmap merges across rtgroup boundaries

Source kernel commit: 8458c4944e10aa8119d9de88e257d60a3537263e

Except for the rt superblock, realtime groups do not store any metadata
at the start (or end) of the group.  There is nothing to prevent the
bmap code from merging allocations from multiple groups into a single
bmap record.  Add a helper to check for this case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: massage the commit message after pulling this into rtgroups]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>