backport sched_rt_period_us and sysctl_sched_rr_timeslice invalid values fixes

Hi,

The commit `c1fc6484e1fb sched/rt: sysctl_sched_rr_timeslice show default timeslice after reset` is a clean cherry-pick for v5.4+ kernels

and the commit `079be8fc6309 sched/rt: sysctl_sched_rr_timeslice show default timeslice after reset` cleanly applicable for v6.1

These are trivial fixes which fix the parsing the negative values when read from the userspace, but will also make LTP test `proc_sched_rt01.c` happy instead of ignoring it.

-MNAdam