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/pipes/fifo.c

@@ -47,8 +47,8 @@ static const struct file_operations g_fifo_fops =
   pipecommon_write,    /* write */
   NULL,                /* seek */
   pipecommon_ioctl,    /* ioctl */
-  NULL,                /* truncate */
   NULL,                /* mmap */
+  NULL,                /* truncate */
   pipecommon_poll      /* poll */
 #ifndef CONFIG_DISABLE_PSEUDOFS_OPERATIONS
   , pipecommon_unlink  /* unlink */

drivers/pipes/pipe.c

@@ -61,8 +61,8 @@ static const struct file_operations g_pipe_fops =
   pipecommon_write,    /* write */
   NULL,                /* seek */
   pipecommon_ioctl,    /* ioctl */
-  NULL,                /* truncate */
   NULL,                /* mmap */
+  NULL,                /* truncate */
   pipecommon_poll      /* poll */
 };