[IPV6]: Remove redundant length check on input

We don't need to check skb->len when we're just about to call
pskb_may_pull since that checks it for us.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>