[MIPS] Fix check for valid stack pointer during backtrace

The newly added check for valid stack pointer address breaks at least for
64bit kernels.  Use __get_user() for accessing stack content to avoid crashes,
when doing the backtrace.
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/traps.c

@@ -88,15 +88,17 @@ static void show_raw_backtrace(unsigned long reg29)
 #ifdef CONFIG_KALLSYMS
 	printk("\n");
 #endif
-#define IS_KVA01(a) ((((unsigned int)a) & 0xc0000000) == 0x80000000)
-	if (IS_KVA01(sp)) {
 	while (!kstack_end(sp)) {
-			addr = *sp++;
+		unsigned long __user *p =
+			(unsigned long __user *)(unsigned long)sp++;
+		if (__get_user(addr, p)) {
+			printk(" (Bad stack address)");
+			break;
+		}
 		if (__kernel_text_address(addr))
 			print_ip_sym(addr);
 	}
 	printk("\n");
-	}
 }
 
 #ifdef CONFIG_KALLSYMS