xfs: move xfs_rtget_summary to xfs_rtbitmap.c

xfs_rtmodify_summary_int is only used inside xfs_rtbitmap.c and to
implement xfs_rtget_summary.  Move xfs_rtget_summary to xfs_rtbitmap.c
as the exported API and mark xfs_rtmodify_summary_int static.

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>