net: warn if NAPI instance wasn't shut down

Drivers should always disable a NAPI instance before removing it.
If they don't the instance may be queued for polling.
Since commit 86e25f4 ("net: napi: Add napi_config")
we also remove the NAPI from the busy polling hash table
in napi_disable(), so not disabling would leave a stale
entry there.

Use of busy polling is relatively uncommon so bugs may be lurking
in the drivers. Add an explicit warning.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250203215816.1294081-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -7071,6 +7071,9 @@ void __netif_napi_del_locked(struct napi_struct *napi)
 	if (!test_and_clear_bit(NAPI_STATE_LISTED, &napi->state))
 		return;
 
+	/* Make sure NAPI is disabled (or was never enabled). */
+	WARN_ON(!test_bit(NAPI_STATE_SCHED, &napi->state));
+
 	if (napi->config) {
 		napi->index = -1;
 		napi->config = NULL;