printk: Fail pr_flush() if before SYSTEM_SCHEDULING

A follow-up change adds pr_flush() to console unregistration.
However, with boot consoles unregistration can happen very
early if there are also regular consoles registering as well.
In this case the pr_flush() is not important because all
consoles are flushed when checking the initial console sequence
number.

Allow pr_flush() to fail if @system_state has not yet reached
SYSTEM_SCHEDULING. This avoids might_sleep() and msleep()
explosions that would otherwise occur:

[    0.436739][    T0] printk: legacy console [ttyS0] enabled
[    0.439820][    T0] printk: legacy bootconsole [earlyser0] disabled
[    0.446822][    T0] BUG: scheduling while atomic: swapper/0/0/0x00000002
[    0.450491][    T0] 1 lock held by swapper/0/0:
[    0.457897][    T0]  #0: ffffffff82ae5f88 (console_mutex){+.+.}-{4:4}, at: console_list_lock+0x20/0x70
[    0.463141][    T0] Modules linked in:
[    0.465307][    T0] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.10.0-rc1+ #372
[    0.469394][    T0] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
[    0.474402][    T0] Call Trace:
[    0.476246][    T0]  <TASK>
[    0.481473][    T0]  dump_stack_lvl+0x93/0xb0
[    0.483949][    T0]  dump_stack+0x10/0x20
[    0.486256][    T0]  __schedule_bug+0x68/0x90
[    0.488753][    T0]  __schedule+0xb9b/0xd80
[    0.491179][    T0]  ? lock_release+0xb5/0x270
[    0.493732][    T0]  schedule+0x43/0x170
[    0.495998][    T0]  schedule_timeout+0xc5/0x1e0
[    0.498634][    T0]  ? __pfx_process_timeout+0x10/0x10
[    0.501522][    T0]  ? msleep+0x13/0x50
[    0.503728][    T0]  msleep+0x3c/0x50
[    0.505847][    T0]  __pr_flush.constprop.0.isra.0+0x56/0x500
[    0.509050][    T0]  ? _printk+0x58/0x80
[    0.511332][    T0]  ? lock_is_held_type+0x9c/0x110
[    0.514106][    T0]  unregister_console_locked+0xe1/0x450
[    0.517144][    T0]  register_console+0x509/0x620
[    0.519827][    T0]  ? __pfx_univ8250_console_init+0x10/0x10
[    0.523042][    T0]  univ8250_console_init+0x24/0x40
[    0.525845][    T0]  console_init+0x43/0x210
[    0.528280][    T0]  start_kernel+0x493/0x980
[    0.530773][    T0]  x86_64_start_reservations+0x18/0x30
[    0.533755][    T0]  x86_64_start_kernel+0xae/0xc0
[    0.536473][    T0]  common_startup_64+0x12c/0x138
[    0.539210][    T0]  </TASK>

And then the kernel goes into an infinite loop complaining about:

1. releasing a pinned lock
2. unpinning an unpinned lock
3. bad: scheduling from the idle thread!
4. goto 1
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-3-john.ogness@linutronix.deSigned-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/printk.c

@@ -3991,6 +3991,10 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 	u64 diff;
 	u64 seq;
 
+	/* Sorry, pr_flush() will not work this early. */
+	if (system_state < SYSTEM_SCHEDULING)
+		return false;
+
 	might_sleep();
 
 	seq = prb_next_reserve_seq(prb);