mlx4_core: Clean up struct mlx4_buf

Now that struct mlx4_buf.u is a struct instead of a union because of
the vmap() changes, there's no point in having a struct at all.  So
move .direct and .page_list directly into struct mlx4_buf and get rid
of a bunch of unnecessary ".u"s.

Signed-off-by: Roland Dreier <rolandd@cisco.com>