lib/nmi_backtrace: explicitly serialize banner and regs

Currently the nmi_backtrace is serialized against other CPUs because
the messages are sent to the NMI buffers. Once these buffers are
removed, only the dumped stack will be serialized against other CPUs
(via the printk_cpu_lock).

Also serialize the nmi_backtrace banner and regs using the
printk_cpu_lock so that per-CPU serialization will be preserved even
after the NMI buffers are removed.

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/20210715193359.25946-2-john.ogness@linutronix.de