MIPS: Fix Magic SysRq L kernel crash.

show_backtrace() was passed a NULL pointer which caused paging
request fail. Set to current task as other architectures (ARM,
etc) do when passed a NULL task pointer.
Signed-off-by: Vincent Wen <vincentwenlinux@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/3524/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/traps.c

@@ -132,6 +132,9 @@ static void show_backtrace(struct task_struct *task, const struct pt_regs *regs)
 	unsigned long ra = regs->regs[31];
 	unsigned long pc = regs->cp0_epc;
 
+	if (!task)
+		task = current;
+
 	if (raw_show_trace || !__kernel_text_address(pc)) {
 		show_raw_backtrace(sp);
 		return;