xfs: split xfs_rtmodify_summary_int

Inline the logic of xfs_rtmodify_summary_int into xfs_rtmodify_summary
and xfs_rtget_summary instead of having a somewhat awkward helper to
share a little bit of code.

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