sched: Disallow sched_attr::sched_policy < 0

The scheduler uses policy=-1 to preserve the current policy state to
implement sys_sched_setparam(), this got exposed to userspace by
accident through sys_sched_setattr(), cure this.

Reported-by: Michael Kerrisk <[e-mail]>
Signed-off-by: Peter Zijlstra <[e-mail]>
Acked-by: Michael Kerrisk <[e-mail]>
Cc: <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/sched/core.c

···
 	if (retval)
 		return retval;
 
+	if (attr.sched_policy < 0)
+		return -EINVAL;
+
 	rcu_read_lock();
 	retval = -ESRCH;
 	p = find_process_by_pid(pid);