ipv6: fix possible crashes in ip6_cork_release()

commit 0178b695 ("ipv6: Copy cork options in ip6_append_data")
added some code duplication and bad error recovery, leading to potential
crash in ip6_cork_release() as kfree() could be called with garbage.

use kzalloc() to make sure this wont happen.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Neal Cardwell <ncardwell@google.com>

net/ipv6/ip6_output.c

@@ -1147,7 +1147,7 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to,
 			if (WARN_ON(np->cork.opt))
 				return -EINVAL;
 
-			np->cork.opt = kmalloc(opt->tot_len, sk->sk_allocation);
+			np->cork.opt = kzalloc(opt->tot_len, sk->sk_allocation);
 			if (unlikely(np->cork.opt == NULL))
 				return -ENOBUFS;