cgroup: duplicate cgroup reference when cloning sockets

cgroup: duplicate cgroup reference when cloning sockets

When a socket is cloned, the associated sock_cgroup_data is duplicated
but not its reference on the cgroup.  As a result, the cgroup reference
count will underflow when both sockets are destroyed later on.

Fixes: bd1060a1d671 ("sock, cgroup: add sock->sk_cgroup")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Johannes Weiner <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: <[email protected]>	[4.5+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>