ipvs: Fix possible deadlock in estimator code

There is a slight chance for a deadlock in the estimator code. We can't call
del_timer_sync() while holding our lock, as the timer might be active and
spinning for the lock on another cpu. Work around this issue by using
try_to_del_timer_sync() and releasing the lock. We could actually delete the
timer outside of our lock, as the add and kill functions are only every called
from userspace via [gs]etsockopt() and are serialized by a mutex, but better
make this explicit.
Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Cc: stable <stable@kernel.org>
Acked-by: Simon Horman <horms@verge.net.au>

net/ipv4/ipvs/ip_vs_est.c

@@ -170,8 +170,11 @@ void ip_vs_kill_estimator(struct ip_vs_stats *stats)
 		kfree(est);
 		killed++;
 	}
-	if (killed && est_list == NULL)
-		del_timer_sync(&est_timer);
+	while (killed && !est_list && try_to_del_timer_sync(&est_timer) < 0) {
+		write_unlock_bh(&est_lock);
+		cpu_relax();
+		write_lock_bh(&est_lock);
+	}
 	write_unlock_bh(&est_lock);
 }