[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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Haavard Skinnemoen <[e-mail]>

arch/avr32/mm/fault.c

···
 	up_read(&mm->mmap_sem);
 
 	if (user_mode(regs)) {
-		if (exception_trace)
+		if (exception_trace && printk_ratelimit())
 			printk("%s%s[%d]: segfault at %08lx pc %08lx "
 			       "sp %08lx ecr %lu\n",
 			       is_init(tsk) ? KERN_EMERG : KERN_INFO,