Fix build break in fork.c when THREAD_SIZE < PAGE_SIZE

Commit b235beea9e99 ("Clarify naming of thread info/stack allocators")
breaks the build on some powerpc configs, where THREAD_SIZE < PAGE_SIZE:

  kernel/fork.c:235:2: error: implicit declaration of function 'free_thread_stack'
  kernel/fork.c:355:8: error: assignment from incompatible pointer type
    stack = alloc_thread_stack_node(tsk, node);
    ^

Fix it by renaming free_stack() to free_thread_stack(), and updating the
return type of alloc_thread_stack_node().

Fixes: b235beea9e99 ("Clarify naming of thread info/stack allocators")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -183,13 +183,13 @@ static inline void free_thread_stack(unsigned long *stack)
 # else
 static struct kmem_cache *thread_stack_cache;
 
-static struct thread_info *alloc_thread_stack_node(struct task_struct *tsk,
+static unsigned long *alloc_thread_stack_node(struct task_struct *tsk,
 						  int node)
 {
 	return kmem_cache_alloc_node(thread_stack_cache, THREADINFO_GFP, node);
 }
 
-static void free_stack(unsigned long *stack)
+static void free_thread_stack(unsigned long *stack)
 {
 	kmem_cache_free(thread_stack_cache, stack);
 }