parisc: ratelimit userspace segfault printing

Ratelimit printing of userspace segfaults and make it runtime
configurable via the /proc/sys/debug/exception-trace variable. This
should resolve syslog from growing way too fast and thus prevents
possible system service attacks.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # 3.13+

diff --cc arch/parisc/Kconfig
Simple merge

diff --cc arch/parisc/kernel/traps.c
Simple merge

diff --cc arch/parisc/mm/fault.c
Simple merge