Add a function to determine cow op size

This is useful for update-engine to run on-device size estimates.
For example, when user wants to turn off compression parameter
on demand.

Test: th
Change-Id: I1e9cca88dff2492feeada3edcf0da2eb5f9430cc
diff --git a/fs_mgr/libsnapshot/include/libsnapshot/cow_format.h b/fs_mgr/libsnapshot/include/libsnapshot/cow_format.h
index 991e17c..66f9a83 100644
--- a/fs_mgr/libsnapshot/include/libsnapshot/cow_format.h
+++ b/fs_mgr/libsnapshot/include/libsnapshot/cow_format.h
@@ -329,6 +329,16 @@
     uint8_t read_ahead_state;
 } __attribute__((packed));
 
+constexpr size_t GetCowOpSize(size_t version) {
+    if (version == 3) {
+        return sizeof(CowOperationV3);
+    } else if (version == 2 || version == 1) {
+        return sizeof(CowOperationV2);
+    } else {
+        return 0;
+    }
+}
+
 // 2MB Scratch space used for read-ahead
 static constexpr uint64_t BUFFER_REGION_DEFAULT_SIZE = (1ULL << 21);