sched/rt: Change default SCHED_RR timeslice from 100 ms to 1 jiffy

For us, it's most helpful to have the round-robin timeslice as low as is
allowed by the scheduler to reduce latency. Since it's limited by the
scheduler tick rate, just set the default to 1 jiffy, which is the
lowest possible value.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>