fix: compatibility to image without ext table for blob cache

    There are scenes that cache file is smaller than expect size. Such as:

    1. Nydusd 1.6 generates cache file by prefetch, which is smaller than size in boot.
    2. Nydusd 2.2 generates cache file by prefetch, when image not provide ext blob tables.
    3. Nydusd not have enough time to fill cache for blob.

    Equality check for size is too much strict for both 1.6
    compatibility and 2.2 concurrency. This pr ensures blob size smaller
    or equal than expect size. It also truncates blob cache when smaller
    than expect size.

Signed-off-by: 泰友 <cuichengxu.ccx@antgroup.com>

storage/src/cache/filecache/mod.rs

@@ -257,7 +257,7 @@ impl FileCacheEntry {
             } else {
                 blob_info.uncompressed_size()
             };
-            if file_size == 0 {
+            if file_size == 0 || file_size < cached_file_size {
                 file.set_len(cached_file_size)?;
             } else if cached_file_size != 0 && file_size != cached_file_size {
                 let msg = format!(