printk: introduce kernel sync mode

When the kernel performs an OOPS, enter into "sync mode":

- only atomic consoles (write_atomic() callback) will print
- printing occurs within vprintk_store() instead of console_unlock()

CONSOLE_LOG_MAX is moved to printk.h to support the per-console
buffer used in sync mode.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>