ARC: kernel stack: INIT_THREAD need not setup @init_stack in @ksp

ARC: kernel stack: INIT_THREAD need not setup @init_stack in @ksp

There are 2 pointers to kernel mode stack of a task
 - task_struct.stack: base address of stack page (max possible stack top)
 - thread_info.ksp  : runtime stack top in __switch_to

INIT_THREAD was setting up ksp to stack base which was not really needed
 - it would get overwritten with dynamic value on first call to
   __switch_to when init is switched out for the very first time.
 - generic code already does
      init_task.stack = init_stack
   and ARC code uses that to retrieve task's stack base.

Signed-off-by: Vineet Gupta <vgupta@kernel.org>