fix up initial thread stack pointer vs thread_info confusion

The INIT_TASK() initializer was similarly confused about the stack vs
thread_info allocation that the allocators had, and that were fixed in
commit b235beea9e ("Clarify naming of thread info/stack allocators").

The task ->stack pointer only incidentally ends up having the same value
as the thread_info, and in fact that will change.

So fix the initial task struct initializer to point to 'init_stack'
instead of 'init_thread_info', and make sure the ia64 definition for
that exists.

This actually makes the ia64 tsk->stack pointer be sensible for the
initial task, but not for any other task.  As mentioned in commit
b235beea9e, that whole pointer isn't actually used on ia64, since
task_stack_page() there just points to the (single) allocation.

All the other architectures seem to have copied the 'init_stack'
definition, even if it tended to be generally unusued.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/init_task.h

@@ -190,7 +190,7 @@ extern struct task_group root_task_group;
 #define INIT_TASK(tsk)	\
 {									\
 	.state		= 0,						\
-	.stack		= &init_thread_info,				\
+	.stack		= init_stack,					\
 	.usage		= ATOMIC_INIT(2),				\
 	.flags		= PF_KTHREAD,					\
 	.prio		= MAX_PRIO-20,					\