net, bpf: Add a warning if NAPI cb missed xdp_do_flush().

A few drivers were missing a xdp_do_flush() invocation after
XDP_REDIRECT.

Add three helper functions each for one of the per-CPU lists. Return
true if the per-CPU list is non-empty and flush the list.

Add xdp_do_check_flushed() which invokes each helper functions and
creates a warning if one of the functions had a non-empty list.

Hide everything behind CONFIG_DEBUG_NET.

Suggested-by: Jesper Dangaard Brouer <hawk@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20231016125738.Yt79p1uF@linutronix.de

kernel/bpf/devmap.c

@@ -418,6 +418,16 @@ void __dev_flush(void)
 	}
 }
 
+#ifdef CONFIG_DEBUG_NET
+bool dev_check_flush(void)
+{
+	if (list_empty(this_cpu_ptr(&dev_flush_list)))
+		return false;
+	__dev_flush();
+	return true;
+}
+#endif
+
 /* Elements are kept alive by RCU; either by rcu_read_lock() (from syscall) or
  * by local_bh_disable() (from XDP calls inside NAPI). The
  * rcu_read_lock_bh_held() below makes lockdep accept both.