It's redundancy for the drivers to hold the list of vlans when
absolutely the same list exists in vlan core. In most cases it's
needed only to traverse the vlan devices, their vids and sync some
settings with h/w, so add API to simplify this.

At least some of these drivers also can benefit:
grep "for_each.*vid" -r drivers/net/ethernet/

drivers/net/ethernet/hisilicon/hns3/hns3_enet.c:
drivers/net/ethernet/synopsys/dwc-xlgmac-hw.c:
drivers/net/ethernet/qlogic/qlge/qlge_main.c:
drivers/net/ethernet/qlogic/qlcnic/qlcnic_main.c:
drivers/net/ethernet/via/via-rhine.c:
drivers/net/ethernet/via/via-velocity.c:
drivers/net/ethernet/intel/igb/igb_main.c:
drivers/net/ethernet/intel/ice/ice_main.c:
drivers/net/ethernet/intel/e1000/e1000_main.c:
drivers/net/ethernet/intel/i40e/i40e_main.c:
drivers/net/ethernet/intel/e1000e/netdev.c:
drivers/net/ethernet/intel/igbvf/netdev.c:
drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c:
drivers/net/ethernet/intel/ixgb/ixgb_main.c:
drivers/net/ethernet/intel/ixgbe/ixgbe_main.c:
drivers/net/ethernet/amd/xgbe/xgbe-dev.c:
drivers/net/ethernet/emulex/benet/be_main.c:
drivers/net/ethernet/neterion/vxge/vxge-main.c:
drivers/net/ethernet/adaptec/starfire.c:
drivers/net/ethernet/brocade/bna/bnad.c:
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>