When task_struct was moved, this MIPS code was neglected. Evidently
nobody is using it anymore. This fixes this build error:

In file included from ./arch/mips/include/asm/thread_info.h:15:0,
                 from ./include/linux/thread_info.h:37,
                 from ./include/asm-generic/current.h:4,
                 from ./arch/mips/include/generated/asm/current.h:1,
                 from ./include/linux/sched.h:11,
                 from arch/mips/kernel/smp-cmp.c:22:
arch/mips/kernel/smp-cmp.c: In function ‘cmp_boot_secondary’:
./arch/mips/include/asm/processor.h:384:41: error: implicit declaration
of function ‘task_stack_page’ [-Werror=implicit-function-declaration]
 #define __KSTK_TOS(tsk) ((unsigned long)task_stack_page(tsk) + \
                                         ^
arch/mips/kernel/smp-cmp.c:84:21: note: in expansion of macro ‘__KSTK_TOS’
  unsigned long sp = __KSTK_TOS(idle);
                     ^~~~~~~~~~

Fixes: f3ac6067 ("sched/headers: Move task-stack related APIs from <linux/sched.h> to <linux/sched/task_stack.h>")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: <stable@vger.kernel.org> # 4.11+
Patchwork: https://patchwork.linux-mips.org/patch/17522/Signed-off-by: James Hogan <jhogan@kernel.org>