net: 8021q: move vlan offload registrations into vlan_core

Currently, the vlan packet offloads are registered only upon 8021q module
load. However, even without this module loaded, the offloads could be
utilized, for example by openvswitch datapath. As reported by Michael,
that causes 2x to 5x performance improvement, depending on a testcase.

So move the vlan offload registrations into vlan_core and make this
available even without 8021q module loaded.

Reported-by: Michael Shteinbok <michaelsh86@gmail.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Tested-by: Michael Shteinbok <michaelsh86@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>