Fix race between sk_filter reassign and sk_clone()

The race can result in that some sock will get an sk_filter pointer set to kfree-d memory. Look

CPU1:                            CPU2:
sk_clone():                      sk_attach_filter():
  new_sk = sk_alloc(...);
  sock_copy(new_sk, sk); 
  /* copies the filter ptr */
  ...
  filter = new_sk->sk_filter;
  if (filter)
                                     old_fp = sk->sk_filter;
                                     ...
                                     sk_filter_release(old_fp);
                                       if (atomic_dec_and_test(&old_fp->refcnt))
     atomic_inc(&filter->refcnt);
                                          /* true */
                                          call_rcu(&fp->rcu, kfree);

that's it - after a quiescent state pass the new_sk will have a pointer on kfree-d filter.

The same problem exists for detaching filter (SO_DETACH_FILTER).

The proposed fix consists of 3 preparation patches and the fix itself.