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>

arch/m68k/include/asm/cmpxchg.h

@@ -76,7 +76,7 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz
 }
 #endif
 
-#define xchg(ptr,x) ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))
+#define xchg(ptr,x) ({(__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr)));})
 
 #include <asm-generic/cmpxchg-local.h>
 
@@ -119,11 +119,11 @@ static inline unsigned long __cmpxchg(volatile void *p, unsigned long old,
 }
 
 #define cmpxchg(ptr, o, n)						    \
-	((__typeof__(*(ptr)))__cmpxchg((ptr), (unsigned long)(o),	    \
-			(unsigned long)(n), sizeof(*(ptr))))
+	({(__typeof__(*(ptr)))__cmpxchg((ptr), (unsigned long)(o),	    \
+			(unsigned long)(n), sizeof(*(ptr)));})
 #define cmpxchg_local(ptr, o, n)					    \
-	((__typeof__(*(ptr)))__cmpxchg((ptr), (unsigned long)(o),	    \
-			(unsigned long)(n), sizeof(*(ptr))))
+	({(__typeof__(*(ptr)))__cmpxchg((ptr), (unsigned long)(o),	    \
+			(unsigned long)(n), sizeof(*(ptr)));})
 
 #define cmpxchg64(ptr, o, n)	cmpxchg64_local((ptr), (o), (n))