seccomp: allow sched_setscheduler() with SCHED_IDLE policy

seccomp: allow sched_setscheduler() with SCHED_IDLE policy

Current and upcoming mesa releases rely on a shader disk cash. It uses
a thread job queue with low priority, set with
sched_setscheduler(SCHED_IDLE). However, that syscall is rejected by
the "resourcecontrol" seccomp qemu filter.

Since it should be safe to allow lowering thread priority, let's allow
scheduling thread to idle policy.

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1594456

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Eduardo Otubo <otubo@redhat.com>