net neigh: neigh_delete() and neigh_add() changes

neigh_delete() and neigh_add() dont need to touch device refcount,
we hold RTNL when calling them, so device cannot disappear under us.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>