Write CowReplace blocks in batch

snapuserd is optimizing merge performance, sending blocks in batch will
help.

Test: th
Change-Id: Ie358137768b1f0d7e03530e5f1ddb57e40e183ed
diff --git a/common/cow_operation_convert.h b/common/cow_operation_convert.h
index c0543f7..60c820f 100644
--- a/common/cow_operation_convert.h
+++ b/common/cow_operation_convert.h
@@ -30,8 +30,9 @@
     CowReplace = android::snapshot::kCowReplaceOp,
   };
   Type op;
-  uint64_t src_block;
-  uint64_t dst_block;
+  uint64_t src_block{};
+  uint64_t dst_block{};
+  uint64_t block_count{1};
 };
 
 // Convert SOURCE_COPY operations in `operations` list to a list of