net: rps: fix error when CONFIG_RFS_ACCEL is off

John Sperbeck reported that if we turn off CONFIG_RFS_ACCEL, the 'head'
is not defined, which will trigger compile error. So I move the 'head'
out of the CONFIG_RFS_ACCEL scope.

Fixes: 84b6823 ("net: rps: protect last_qtail with rps_input_queue_tail_save() helper")
Reported-by: John Sperbeck <jsperbeck@google.com>
Closes: https://lore.kernel.org/all/20240529203421.2432481-1-jsperbeck@google.com/
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240530032717.57787-1-kerneljasonxing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: JasonXing

net/core/dev.c

@@ -4516,12 +4516,13 @@ set_rps_cpu(struct net_device *dev, struct sk_buff *skb,
 	    struct rps_dev_flow *rflow, u16 next_cpu)
 {
 	if (next_cpu < nr_cpu_ids) {
+		u32 head;
 #ifdef CONFIG_RFS_ACCEL
 		struct netdev_rx_queue *rxqueue;
 		struct rps_dev_flow_table *flow_table;
 		struct rps_dev_flow *old_rflow;
-		u32 flow_id, head;
 		u16 rxq_index;
+		u32 flow_id;
 		int rc;
 
 		/* Should we steer this flow to a different hardware queue? */