xfs_db: expose the unwritten flag in rmapbt keys

Teach the debugger to expose the "unwritten" flag in rmapbt keys so that
we can simulate an old filesystem writing out bad keys for testing.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>