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 !

Signed-off-by: Eric Dumazet <[email protected]>
Reported-by: Pray3r <[email protected]>
Reported-by: Andrey Konovalov <[email protected]>
Tested-by: Andrey Konovalov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>