smp: Make symbol 'csd_bug_count' static

The sparse tool complains as follows:

kernel/smp.c:107:10: warning:
 symbol 'csd_bug_count' was not declared. Should it be static?

Because variable is not used outside of smp.c, this commit marks it
static.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

kernel/smp.c

@@ -106,7 +106,7 @@ static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
 static DEFINE_PER_CPU(void *, cur_csd_info);
 
 #define CSD_LOCK_TIMEOUT (5ULL * NSEC_PER_SEC)
-atomic_t csd_bug_count = ATOMIC_INIT(0);
+static atomic_t csd_bug_count = ATOMIC_INIT(0);
 
 /* Record current CSD work for current CPU, NULL to erase. */
 static void csd_lock_record(call_single_data_t *csd)