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>

fs/smartfs/smartfs_smart.c

@@ -140,8 +140,8 @@ const struct mountpt_operations smartfs_operations =
   smartfs_write,         /* write */
   smartfs_seek,          /* seek */
   smartfs_ioctl,         /* ioctl */
-  smartfs_truncate,      /* truncate */
   NULL,                  /* mmap */
+  smartfs_truncate,      /* truncate */
 
   smartfs_sync,          /* sync */
   smartfs_dup,           /* dup */