xfs: add a lockdep class key for rtgroup inodes

Add a dynamic lockdep class key for rtgroup inodes.  This will enable
lockdep to deduce inconsistencies in the rtgroup metadata ILOCK locking
order.  Each class can have 8 subclasses, and for now we will only have
2 inodes per group.  This enables rtgroup order and inode order checks
when nesting ILOCKs.

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