um/Stop conflating task_struct::stack with thread_info

thread_info may move in the future, so use the accessors.

[ Andy Lutomirski wrote this changelog message and changed
  "task_thread_info(child)->cpu" to "task_cpu(child)". ]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jann Horn <jann@thejh.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/3439705d9838940cc82733a7335fa8c654c37db8.1473801993.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>