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>