sched: Add helper kstat_cpu_softirqs_sum()

Add a kstat_cpu_softirqs_sum() function that is similar to
kstat_cpu_irqs_sum(), but which counts software interrupts since boot
on the specified CPU.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Cc: Josh Don <joshdon@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>