kasan: add a prototype of task_struct to avoid warning

Add a prototype of task_struct to fix below warning on arm64.

  In file included from arch/arm64/kernel/probes/kprobes.c:19:0:
  include/linux/kasan.h:81:132: error: 'struct task_struct' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
   static inline void kasan_unpoison_task_stack(struct task_struct *task) {}

As same as other types (kmem_cache, page, and vm_struct) this adds a
prototype of task_struct data structure on top of kasan.h.

[arnd] A related warning was fixed before, but now appears in a
different line in the same file in v4.11-rc2.  The patch from Masami
Hiramatsu still seems appropriate, so let's take his version.

Fixes: 71af2ed ("kasan, sched/headers: Remove <linux/sched.h> from <linux/kasan.h>")
Link: https://patchwork.kernel.org/patch/9569839/
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Masami Hiramatsu <[email protected]>
Acked-by: Alexander Potapenko <[email protected]>
Acked-by: Andrey Ryabinin <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/kasan.h

@@ -6,6 +6,7 @@
 struct kmem_cache;
 struct page;
 struct vm_struct;
+struct task_struct;
 
 #ifdef CONFIG_KASAN