neigh: use NEIGH_VAR_INIT in ndo_neigh_setup functions.

When ndo_neigh_setup is called, the bitfield used by NEIGH_VAR_SET is
not initialized yet. This might cause confusion for the people who use
NEIGH_VAR_SET in ndo_neigh_setup. So rather introduce NEIGH_VAR_INIT for
usage in ndo_neigh_setup.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>