dccp/tcp: do not inherit mc_list from parent

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 !

Bug 1971958

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>
Change-Id: I49a3d7de687451ead042783517f944fb3c6e5c6e
Reviewed-on: https://git-master.nvidia.com/r/1560672
GVS: Gerrit_Virtual_Submit
Reviewed-by: Hayden Du <haydend@nvidia.com>
Tested-by: Hayden Du <haydend@nvidia.com>
Reviewed-by: James Huang <jamehuang@nvidia.com>
Tested-by: James Huang <jamehuang@nvidia.com>