sched: Fix preemption string of preempt_dynamic_none

Zero is a valid value for "preempt_dynamic_mode", namely
"preempt_dynamic_none".

Fix the off-by-one in preempt_model_str(), so that "preempty_dynamic_none"
is correctly formatted as PREEMPT(none) instead of PREEMPT(undef).

Fixes: 8bdc5da ("sched: Add a generic function to return the preemption string")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20250626-preempt-str-none-v2-1-526213b70a89@linutronix.de

Author: t-8ch

kernel/sched/core.c

@@ -7663,7 +7663,7 @@ const char *preempt_model_str(void)
 
 		if (IS_ENABLED(CONFIG_PREEMPT_DYNAMIC)) {
 			seq_buf_printf(&s, "(%s)%s",
-				       preempt_dynamic_mode > 0 ?
+				       preempt_dynamic_mode >= 0 ?
 				       preempt_modes[preempt_dynamic_mode] : "undef",
 				       brace ? "}" : "");
 			return seq_buf_str(&s);