net: trivial cast fix in ipv4/route.c

To allow macros pick the right implementation cast to (spinlock_t *).
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

net/ipv4/route.c

@@ -242,7 +242,7 @@ static __init void rt_hash_lock_init(void)
 		spin_lock_init(&rt_hash_locks[i]);
 }
 #else
-# define rt_hash_lock_addr(slot) NULL
+# define rt_hash_lock_addr(slot) ((spinlock_t *)NULL)
 
 static inline void rt_hash_lock_init(void)
 {