xfs: convert unwritten status of reverse mappings

Provide a function to convert an unwritten rmap extent to a real one
and vice versa.

[ dchinner: Note that this algorithm and code was derived from the
  existing bmapbt unwritten extent conversion code in
  xfs_bmap_add_extent_unwritten_real(). ]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>