ipv6: mip6: fix mip6_mh_filter()

mip6_mh_filter() should not modify its input, or else its caller
would need to recompute ipv6_hdr() if skb->head is reallocated.

Use skb_header_pointer() instead of pskb_may_pull()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>