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/romfs/fs_romfs.c

@@ -121,8 +121,8 @@ const struct mountpt_operations romfs_operations =
   NULL,            /* write */
   romfs_seek,      /* seek */
   romfs_ioctl,     /* ioctl */
-  NULL,            /* truncate */
   romfs_mmap,      /* mmap */
+  NULL,            /* truncate */
 
   NULL,            /* sync */
   romfs_dup,       /* dup */