vlan: Add function to retrieve EtherType from vlan packets.
Depending on how a packet is vlan tagged (i.e. hardware accelerated or
not), the encapsulated protocol is stored in different locations. This
provides a consistent method of accessing that protocol, which is needed
by drivers, security checks, etc.
Signed-off-by: Hao Zheng <hzheng@nicira.com> Signed-off-by: Jesse Gross <jesse@nicira.com> Signed-off-by: David S. Miller <davem@davemloft.net>