This patch replaces the skb argument in ipv6_clear_mutable_options() by
an ipv6hdr.  Doing so allows us to point skb->nh elsewhere when calling
this function.

I've also thrown in some obvious clean-ups for that function.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@redhat.com>