sched: do not set softirqs to nice +19

do not set softirqs to nice +19. _If_ for whatever reason
we missed to process some high-prio softirq and woke up
ksoftirqd, we should give it a fair chance to actually
get some work done, even if the system is under load.

Signed-off-by: Ingo Molnar <mingo@elte.hu>