xfs: rename flist/free_list to dfops

Mechanical change of flist/free_list to dfops, since they're now
deferred ops, not just a freeing list.

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