[PATCH] m68k: fix cmpxchg compile errors if CONFIG_RMW_INSNS=n

We require that all archs implement atomic_cmpxchg(), for the generic
version of atomic_add_unless().

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>