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>