net: unpollute priv_flags space

the ipvlan device driver defines and uses 2 bits inside the priv_flags
net_device field. Such bits and the related helper are used only
inside the ipvlan device driver, and the core networking does not
need to be aware of them.

This change moves netif_is_ipvlan* helper in the ipvlan driver and
re-implement them looking for ipvlan specific symbols instead of
using priv_flags.

Overall this frees two bits inside priv_flags - and move the following
ones to avoid gaps - without any intended functional change.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>