printk: wake up all waiters

There can be multiple tasks waiting for new records. They should
all be woken. Use wake_up_interruptible_all() instead of
wake_up_interruptible().

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220421212250.565456-5-john.ogness@linutronix.de