atomic: Allow atomic_inc_not_zero to be overridden

We want to implement a ppc64 specific version of atomic_inc_not_zero
so wrap it in an ifdef to allow it to be overridden.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>