net: Use a dedicated flag to track disabled napi instances

Trying to disable the napi threaded state while the napi instance is
disabled will hang forever in napi_stop_kthread(), as it waits for
NAPIF_STATE_SCHED_THREADED to clear.

This won't happen until the next ____napi_schedule() call, which won't
happen as it's disabled.

This happens in 2 instances :

 - Drivers that create but don't use napi instances (stmmac's rxtx napi
   for AF_XDP for example), here it's a driver bug

 - Drivers that create their napi instances in .probe(). Here, setting
   threaded off while the interface is down will hang :

   ip link set eth0 down
   echo 1 > /sys/class/net/eth0/threaded
   echo 0 > /sys/class/net/eth0/threaded
   -> hang

Checking on netif_running() isn't enough, as napi instances may be
transiently disabled when changing the MTU for example, let's use a
dedicated flag to track if the instance is disabled.

This allows immediately disabling the kthread when the instance is
disabled.

This was tested on mvpp2 (probe-time napi_add) and stmmac.

Fixes: 689883de94dd ("net: stop napi kthreads when THREADED napi is disabled")
Tested-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20261029153130.121204-1-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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