vxlan: fix ND proxy when skb doesn't have transport header offset

When an incoming frame is tagged or when GRO is disabled, the skb
handled to vxlan_xmit() doesn't contain a valid transport header
offset. This makes ND proxying fail.

We combine two changes: replace use of skb_transport_offset() and ensure
the necessary amount of skb is linear just before using it:

 - In vxlan_xmit(), when determining if we have an ICMPv6 neighbor
   discovery packet, just check if it is an ICMPv6 packet and rely on
   neigh_reduce() to do more checks if this is the case. The use of
   pskb_may_pull() is replaced by skb_header_pointer() for just the IPv6
   header.

 - In neigh_reduce(), add pskb_may_pull() for IPv6 header and neighbor
   discovery message since this was removed from vxlan_xmit(). Replace
   skb_transport_header() with ipv6_hdr() + 1.

 - In vxlan_na_create(), replace first skb_transport_offset() with
   ipv6_hdr() + 1 and second with skb_network_offset() + sizeof(struct
   ipv6hdr). Additionally, ensure we pskb_may_pull() the whole skb as we
   need it to iterate over the options.

Signed-off-by: Vincent Bernat <vincent@bernat.im>
Signed-off-by: David S. Miller <davem@davemloft.net>