Migrate is_vlan_dev() to if_vlan.h so that core networkig can use it
Signed-off-by: NNeil Horman <nhorman@tuxdriver.com>
CC: davem@davemloft.net
CC: bhutchings@solarflare.com
Signed-off-by: NDavid S. Miller <davem@davemloft.net>