backtrace: replace timer with tasklet + completions

On qemu, the backtrace would show up _after_ the "end of backtrace
testing" message.

This patch changes it to use completions instead, which will guarantee
that no such race exists.

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