stacktrace: print_stack_trace() cleanup

- shorter code and better atomicity with regards to printk().

(It's been tested with the backtrace self-test code on i386 and x86_64.)

Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>