fork: fix some -Wmissing-prototypes warnings

We get a warning when building kernel with W=1:

  kernel/fork.c:167:13: warning: no previous prototype for `arch_release_thread_stack' [-Wmissing-prototypes]
  kernel/fork.c:779:13: warning: no previous prototype for `fork_init' [-Wmissing-prototypes]

Add the missing declaration in head file to fix this.

Also, remove arch_release_thread_stack() completely because no arch
seems to implement it since bb9d812 (arch: remove tile port).

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yi Wang <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Mike Rapoport <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Yi Wang

include/linux/sched/task.h

@@ -39,6 +39,8 @@ void __noreturn do_task_dead(void);
 
 extern void proc_caches_init(void);
 
+extern void fork_init(void);
+
 extern void release_task(struct task_struct * p);
 
 #ifdef CONFIG_HAVE_COPY_THREAD_TLS

init/main.c

@@ -105,7 +105,6 @@
 static int kernel_init(void *);
 
 extern void init_IRQ(void);
-extern void fork_init(void);
 extern void radix_tree_init(void);
 
 /*

kernel/fork.c

@@ -164,10 +164,6 @@ static inline void free_task_struct(struct task_struct *tsk)
 }
 #endif
 
-void __weak arch_release_thread_stack(unsigned long *stack)
-{
-}
-
 #ifndef CONFIG_ARCH_THREAD_STACK_ALLOCATOR
 
 /*
@@ -422,7 +418,6 @@ static void release_task_stack(struct task_struct *tsk)
 		return;  /* Better to leak the stack than to free prematurely */
 
 	account_kernel_stack(tsk, -1);
-	arch_release_thread_stack(tsk->stack);
 	free_thread_stack(tsk);
 	tsk->stack = NULL;
 #ifdef CONFIG_VMAP_STACK