kernel: only define task_struct_whitelist conditionally

If CONFIG_ARCH_TASK_STRUCT_ALLOCATOR is set task_struct_whitelist is
never called, and thus generates a compiler warning.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/20190812065524.19959-5-hch@lst.deSigned-off-by: Tony Luck <tony.luck@intel.com>

kernel/fork.c

@@ -768,6 +768,7 @@ static void set_max_threads(unsigned int max_threads_suggested)
 int arch_task_struct_size __read_mostly;
 #endif
 
+#ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
 static void task_struct_whitelist(unsigned long *offset, unsigned long *size)
 {
 	/* Fetch thread_struct whitelist for the architecture. */
@@ -782,6 +783,7 @@ static void task_struct_whitelist(unsigned long *offset, unsigned long *size)
 	else
 		*offset += offsetof(struct task_struct, thread);
 }
+#endif /* CONFIG_ARCH_TASK_STRUCT_ALLOCATOR */
 
 void __init fork_init(void)
 {