UPSTREAM: memcg: Only free spare array when readers are done

A spare array holding mem cgroup threshold events is kept around
to make sure we can always safely deregister an event and have an
array to store the new set of events in.

In the scenario where we're going from 1 to 0 registered events, the
pointer to the primary array containing 1 event is copied to the spare
slot, and then the spare slot is freed because no events are left.
However, it is freed before calling synchronize_rcu(), which means
readers may still be accessing threshold->primary after it is freed.

Fixed by only freeing after synchronize_rcu().

Change-Id: Ie56b72e47168d22eba0e4f1af7d8bf3265dc6d47
Signed-off-by: Martijn Coenen <maco@google.com>