xfs: add a xfs_group_next_range helper

xfs: add a xfs_group_next_range helper

Add a helper to iterate over iterate over all groups, which can be used
as a simple while loop:

	struct xfs_group		*xg = NULL;

	while ((xg = xfs_group_next_range(mp, xg, 0, MAX_GROUP))) {
		...
	}

This will be wrapped by the realtime group code first, and eventually
replace the for_each_rtgroup_from and for_each_rtgroup_range helpers.

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>