ipv6: fix possible mem leaks in ipv6_make_skb()

ip6_setup_cork() might return an error, while memory allocations have
been done and must be rolled back.

Fixes: 6422398c

 ("ipv6: introduce ipv6_make_skb")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Reported-by: Mike Maloney <maloney@google.com>
Acked-by: Mike Maloney <maloney@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>