[IA64] Fix futex_atomic_cmpxchg_inatomic()

[IA64] Fix futex_atomic_cmpxchg_inatomic()

Michel Lespinasse cleaned up the futex calling conventions in
commit 37a9d912b24f96a0591773e6e6c3642991ae5a70
    futex: Sanitize cmpxchg_futex_value_locked API

But the ia64 implementation was subtly broken. Gcc does not know
that register "r8" will be updated by the fault handler if the
cmpxchg instruction takes an exception. So it feels safe in letting
the initialization of r8 slide to after the cmpxchg. Result: we
always return 0 whether the user address faulted or not.

Fix by moving the initialization of r8 into the __asm__ code so
gcc won't move it.

Reported-by: <emeric.maschino@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=42757
Cc: stable@vger.kernel.org # v2.6.39+
Signed-off-by: Tony Luck <tony.luck@intel.com>