When combining real_dev's features and vlan_features, simple
bitwise AND is used. This doesn't work well for checksum
offloading features as if one set has NETIF_F_HW_CSUM and the
other NETIF_F_IP_CSUM and/or NETIF_F_IPV6_CSUM, we end up with
no checksum offloading. However, from the logical point of view
(how can_checksum_protocol() works), NETIF_F_HW_CSUM contains
the functionality of NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM so
that the result should be IP/IPV6.

Add helper function netdev_intersect_features() implementing
this logic and use it in vlan_dev_fix_features().

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>