[ARM] 3165/1: fix atomic_cmpxchg() implementation for ARMv6+

Patch from Nicolas Pitre

If 'old' and 'oldval' are different then 'res' never gets set.  In that
case, if ever %0 happened to contain anything but zero (rather likely)
then the code will loop forever (or until another CPU just come along
and change the atomic value to match 'old' which is rather unlikely).

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>