udpv6: Remove unused skb argument of ipv6_select_ident()

- move ipv6_select_ident() inline function to ipv6.h and remove the unused
  skb argument

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>