vhost: fix clang build warning

Clang warns:

drivers/vhost/vhost.c:2085:5: warning: macro expansion producing
'defined' has undefined behavior [-Wexpansion-to-defined]
#if VHOST_ARCH_CAN_ACCEL_UACCESS
^
drivers/vhost/vhost.h:98:38: note: expanded from macro
'VHOST_ARCH_CAN_ACCEL_UACCESS'
#define VHOST_ARCH_CAN_ACCEL_UACCESS defined(CONFIG_MMU_NOTIFIER) && \
^

It's being pedantic for the sake of portability, but the fix is easy
enough.

Rework the definition of VHOST_ARCH_CAN_ACCEL_UACCESS to expand to a constant.

Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
Link: https://github.com/ClangBuiltLinux/linux/issues/508
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Tested-by: Nathan Chancellor <[e-mail]>

drivers/vhost/vhost.h

···
 	bool write;
 };
 
-#define VHOST_ARCH_CAN_ACCEL_UACCESS defined(CONFIG_MMU_NOTIFIER) && \
-	ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE == 0
+#if defined(CONFIG_MMU_NOTIFIER) && ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE == 0
+#define VHOST_ARCH_CAN_ACCEL_UACCESS 1
+#else
+#define VHOST_ARCH_CAN_ACCEL_UACCESS 0
+#endif
 
 /* The virtqueue structure describes a queue attached to a device. */
 struct vhost_virtqueue {