xfs_db: fix rtconvert to handle segmented rtblocks

xfs_db: fix rtconvert to handle segmented rtblocks

Now that we've turned xfs_rtblock_t into a segmented address and
xfs_rtxnum_t into a per-rtgroup address, port the rtconvert debugger
command to handle the unit conversions correctly.  Also add an example
of the bitmap/summary-related conversion commands to the manpage.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>