[AVR32] ratelimit segfault reporting rate

Limit the rate of the kernel logging for the segfaults of user
applications, to avoid potential message floods or denial-of-service
attacks.

Signed-off-by: Andrea Righi <a.righi@cineca.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>