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/sensors/hc_sr04.c

@@ -90,8 +90,8 @@ static const struct file_operations g_hcsr04ops =
   hcsr04_write,  /* write */
   NULL,          /* seek */
   hcsr04_ioctl,  /* ioctl */
-  NULL,          /* truncate */
   NULL,          /* mmap */
+  NULL,          /* truncate */
   hcsr04_poll    /* poll */
 };