ipv6: fix possible crashes in ip6_cork_release()

ipv6: fix possible crashes in ip6_cork_release()

[ Upstream commit 284041ef21fdf2e0d216ab6b787bc9072b4eb58a ]

commit 0178b695fd6b4 ("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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>