fs: Move mmap callback before truncate in [file|mountpt]_operations

since mmap may exist in block_operations, but truncate may not,
moving mmap beforee truncate could make three struct more compatible

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

drivers/usrsock/usrsock_dev.c

@@ -103,8 +103,8 @@ static const struct file_operations g_usrsockdevops =
   usrsockdev_write,   /* write */
   usrsockdev_seek,    /* seek */
   NULL,               /* ioctl */
-  NULL,               /* truncate */
   NULL,               /* mmap */
+  NULL,               /* truncate */
   usrsockdev_poll     /* poll */
 };