lwtunnel: add support for multiple geneve opts

geneve RFC (draft-ietf-nvo3-geneve-14) allows a geneve packet to carry
multiple geneve opts, so it's necessary for lwtunnel to support adding
multiple geneve opts in one lwtunnel route. But vxlan and erspan opts
are still only allowed to add one option.

With this patch, iproute2 could make it like:

  # ip r a 1.1.1.0/24 encap ip id 1 geneve_opts 0:0:12121212,1:2:12121212 \
    dst 10.1.0.2 dev geneve1

  # ip r a 1.1.1.0/24 encap ip id 1 vxlan_opts 456 \
    dst 10.1.0.2 dev erspan1

  # ip r a 1.1.1.0/24 encap ip id 1 erspan_opts 1:123:0:0 \
    dst 10.1.0.2 dev erspan1

Which are pretty much like cls_flower and act_tunnel_key.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>