net: igmp: increase size of mr_ifc_count

Some arches support cmpxchg() on 4-byte and 8-byte only.
Increase mr_ifc_count width to 32bit to fix this problem.

Fixes: 4a2b285e ("net: igmp: fix data-race in igmp_ifc_timer_expire()")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NGuenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210811195715.3684218-1-eric.dumazet@gmail.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

include/linux/inetdevice.h

@@ -41,7 +41,7 @@ struct in_device {
 	unsigned long		mr_qri;		/* Query Response Interval */
 	unsigned char		mr_qrv;		/* Query Robustness Variable */
 	unsigned char		mr_gq_running;
-	unsigned char		mr_ifc_count;
+	u32			mr_ifc_count;
 	struct timer_list	mr_gq_timer;	/* general query timer */
 	struct timer_list	mr_ifc_timer;	/* interface change timer */
 

net/ipv4/igmp.c

@@ -803,7 +803,7 @@ static void igmp_gq_timer_expire(struct timer_list *t)
 static void igmp_ifc_timer_expire(struct timer_list *t)
 {
 	struct in_device *in_dev = from_timer(in_dev, t, mr_ifc_timer);
-	u8 mr_ifc_count;
+	u32 mr_ifc_count;
 
 	igmpv3_send_cr(in_dev);
 restart: