printk: avoid preempt_disable() for PREEMPT_RT

During non-normal operation, printk() calls will attempt to
write the messages directly to the consoles. This involves
using console_trylock() to acquire @console_sem.

Preemption is disabled while directly printing to the consoles
in order to ensure that the printing task is not scheduled away
while holding @console_sem, thus blocking all other printers
and causing delays in printing.

Commit fd5f7cde

 ("printk: Never set console_may_schedule in
console_trylock()") specifically reverted a previous attempt at
allowing preemption while printing.

However, on PREEMPT_RT systems, disabling preemption while
printing is not allowed because console drivers typically
acquire a spin lock (which under PREEMPT_RT is an rtmutex).
Since direct printing is only used during early boot and
non-panic dumps, the risks of delayed print output for these
scenarios will be accepted under PREEMPT_RT.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>