[IA64] Fix stack placement when INIT hits in kernel mode.

Without this patch, the stack is placed _below_ the current task
structure, which is risky at best.

Tony, I think this patch needs to go into 2.6.12, since it fixes a
real bug.  Without it, INIT may case secondary errors, which would be
most unpleasant.

Signed-off-by: David Mosberger-Tang <davidm@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>