builder: fix invalid compressed offset in image.blob toc entry

The `blob_ctx.compressed_offset` has been set to the end of compressed
blob in tar2rafs blob dump, we shouldn't use it as the initial
compressed_offset, this will generate an invalid toc entry.

Signed-off-by: Yan Song <imeoer@linux.alibaba.com>

src/bin/nydus-image/core/blob.rs

@@ -110,7 +110,7 @@ impl Blob {
                         toc::TOC_ENTRY_BLOB_RAW,
                         compress::Algorithm::None,
                         blob_digest,
-                        blob_ctx.compressed_offset,
+                        blob_ctx.begin_compressed_offset(),
                         blob_ctx.compressed_blob_size,
                         blob_ctx.uncompressed_blob_size,
                     )?;

src/bin/nydus-image/core/context.rs

@@ -690,6 +690,12 @@ impl BlobContext {
         self.blob_hash.update(header.as_bytes());
         Ok(header)
     }
+
+    /// Get begin offset of compressed blob, since compressed_offset
+    /// is always >= compressed_blob_size, we can safely subtract here.
+    pub fn begin_compressed_offset(&self) -> u64 {
+        self.compressed_offset - self.compressed_blob_size
+    }
 }
 
 /// BlobManager stores all blob related information during build.