preempt.h: some SOFTIRQ_OFFSET should be SOFTIRQ_MASK?

[Denys Vlasenko]

SOFTIRQ is a counter.
Why here:

#define in_serving_softirq()    (softirq_count() & SOFTIRQ_OFFSET)
#define in_task()               (!(preempt_count() & \
                                   (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))

we check only lowest bit?