x86: split ret_from_fork

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/x86/include/asm/processor.h

@@ -589,11 +589,6 @@ typedef struct {
 } mm_segment_t;
 
 
-/*
- * create a kernel thread without removing it from tasklists
- */
-extern int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
-
 /* Free all resources held by a thread. */
 extern void release_thread(struct task_struct *);