sparc64: Rework xchg() definition to avoid warnings.

Such as:

fs/ocfs2/file.c: In function ‘ocfs2_file_write_iter’:
./arch/sparc/include/asm/cmpxchg_64.h:55:22: warning: value computed is not used [-Wunused-value]
 #define xchg(ptr,x) ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))

and

drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c: In function ‘ixgbevf_xdp_setup’:
./arch/sparc/include/asm/cmpxchg_64.h:55:22: warning: value computed is not used [-Wunused-value]
 #define xchg(ptr,x) ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/include/asm/cmpxchg_64.h

@@ -52,7 +52,12 @@ static inline unsigned long xchg64(__volatile__ unsigned long *m, unsigned long
 	return val;
 }
 
-#define xchg(ptr,x) ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))
+#define xchg(ptr,x)							\
+({	__typeof__(*(ptr)) __ret;					\
+	__ret = (__typeof__(*(ptr)))					\
+		__xchg((unsigned long)(x), (ptr), sizeof(*(ptr)));	\
+	__ret;								\
+})
 
 void __xchg_called_with_bad_pointer(void);