xfs: drop the RTG reference later in xfs_ioc_rtgroup_geometry

xfs: drop the RTG reference later in xfs_ioc_rtgroup_geometry

Keep the rtgroup reference until after reporting the write pointer, as
that uses it.  Right now this is not a major issue as we don't support
shrinking file systems in a way that makes RTGs go away, but let's stick
to the proper reference counting to prepare for that.

Fixes: c6ce65cb17aa ("xfs: add write pointer to xfs_rtgroup_geometry")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>