ipvlan: Fix insufficient skb linear check for ipv6 icmp

In the function ipvlan_get_L3_hdr, current codes use pskb_may_pull to
make sure the skb header has enough linear room for ipv6 header. But it
would use the latter memory directly without linear check when it is icmp.
So it still may access the unepxected memory in ipvlan_addr_lookup.

Now invoke the pskb_may_pull again if it is ipv6 icmp.

Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>