[IA64] Only unwind non-running tasks.

Unwinding a running task has proven problematic.

In one instance, the running task was attempting to unwind itself and
received an interrupt between when get_wchan allocated local variables on
the stack and when unw_init_from_blocked_task was called which resulted
in unw_init_frame_info to place this tasks task_struct pointer over the
switch stack's ar_bspstore entry.

Signed-off-by: Robin Holt <holt@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>