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>