net: mvneta: Fix race condition during stopping

When stopping the port, the CPU notifier are still there whereas the
mvneta_stop_dev function calls mvneta_percpu_disable() on each CPUs.
It was possible to have a new CPU coming at this point which could be
racy.

This patch adds a flag preventing executing the code notifier for a new
CPU when the port is stopping. It also uses the spinlock introduces
previously. To avoid the deadlock, the lock has been moved outside the
mvneta_percpu_elect function.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>