m68k: Add missing __user annotation in get_user()

The ptr is a pointer to userspace memory. So we need annotate it with
__user otherwise we may get sparse warnings like:

drivers/vhost/vhost.c:1603:13: sparse: sparse: incorrect type in initializer (different address spaces) @@    expected void const *__gu_ptr @@    got unsigned int [noderef] [usertypvoid const *__gu_ptr @@
drivers/vhost/vhost.c:1603:13: sparse:    expected void const *__gu_ptr
drivers/vhost/vhost.c:1603:13: sparse:    got unsigned int [noderef] [usertype] <asn:1> *idxp

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20200520065750.8401-1-jasowang@redhat.com
Fixes: 7124330dab ("m68k/uaccess: Revive 64-bit get_user()")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/uaccess_mm.h

@@ -142,7 +142,7 @@ asm volatile ("\n"					\
 		__get_user_asm(__gu_err, x, ptr, u32, l, r, -EFAULT);	\
 		break;							\
 	case 8: {							\
-		const void *__gu_ptr = (ptr);				\
+		const void __user *__gu_ptr = (ptr);			\
 		union {							\
 			u64 l;						\
 			__typeof__(*(ptr)) t;				\