m68k: Avoid xchg() warning

m68k: Avoid xchg() warning

gcc warns about the value of xchg()/cmpxchg() being unused
in some cases:

net/core/filter.c: In function 'bpf_clear_redirect_map':
arch/m68k/include/asm/cmpxchg.h:137:3: warning: value computed is not used [-Wunused-value]
  106 | #define cmpxchg(ptr, o, n) cmpxchg_local((ptr), (o), (n))
net/core/filter.c:3595:4: note: in expansion of macro 'cmpxchg'
 3595 |    cmpxchg(&ri->map, map, NULL);

Shut up that warning like we do on other architectures, by
turning the macro into a statement expression.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20201008123429.1133896-1-arnd@arndb.de
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>