xfs_db: report rt group and block number in the bmap command

xfs_db: report rt group and block number in the bmap command

The bmap command does not report startblocks for realtime files
correctly.  If rtgroups are enabled, we need to use the appropriate
functions to crack the startblock into rtgroup and block numbers; if
not, then we need to report a linear address and not try to report a
group number.

Fix both of these issues.

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