dccp/tcp: do not inherit mc_list from parent

syzkaller found a way to trigger double frees from ip_mc_drop_socket()

It turns out that leave a copy of parent mc_list at accept() time,
which is very bad.

Very similar to commit 8b485ce69876 ("tcp: do not inherit
fastopen_req from parent")

Initial report from Pray3r, completed by Andrey one.
Thanks a lot to them !

Change-Id: Ic896b02962475ca922075674f99ebc03f237193f
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Pray3r <pray3r.z@gmail.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 657831ffc38e30092a2d5f03d385d710eb88b09a
Git-repo: Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>