xfs: make the RT allocator rtgroup aware

xfs: make the RT allocator rtgroup aware

Make the allocator rtgroup aware by either picking a specific group if
there is a hint, or loop over all groups otherwise.  A simple rotor is
provided to pick the placement for initial allocations.

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