[IA64] Tighten up unw_unwind_to_user check

Detect user space by the unwind frame with predicate PRED_USER_STACK
set, instead of a user space IP.  Tighten up the last ditch check for
running off the top of the kernel stack.

Based on a suggestion by David Mosberger, reworked to fit the current
tree.  This survives my stress test which used to break 2.6.9 kernels.
Unlike 2.6.11, the stress test now unwinds to the correct point, so
gdb can get the user space registers.

Signed-off-by: Keith Owens <kaos@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>