[NETNS]: The ip6_fib_timer can work with garbage on net namespace stop.

The del_timer() function doesn't guarantee, that the timer callback
is not active by the time it exits.

Thus, the fib6_net_exit() may kfree() all the data, that is required
by the fib6_run_gc(). The race window is tiny, but slab poisoning can
trigger this bug.

Using del_timer_sync() will cure this.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>