Input: remove useless synchronize_rcu() calls

There is no need to call synchronize_rcu() after a list insertion,
or a NULL->ptr assignment.

However, the reverse operations do need this call.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/input.c

@@ -451,7 +451,6 @@ int input_grab_device(struct input_handle *handle)
 	}
 
 	rcu_assign_pointer(dev->grab, handle);
-	synchronize_rcu();
 
  out:
 	mutex_unlock(&dev->mutex);