xfs: split xfs_trim_rtdev_extents

Split xfs_trim_rtdev_extents into two parts to prepare for reusing the
main validation also for RT group aware file systems.

Use the fully features xfs_daddr_to_rtb helper to convert from a daddr
to a xfs_rtblock_t to prepare for segmented addressing in RT groups.

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>