libsnapshot: move header op count setup

Op count should be set before we sync the header. This way subsequence
writers can initialize with the correct op buffer size

Test: cow_api_test
Change-Id: I56a0d747b3f2a1d9d582d8f9d643b81cbdd9b8d7
diff --git a/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp b/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
index e9e05b0..81ccea9 100644
--- a/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
+++ b/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
@@ -100,6 +100,7 @@
         return false;
     }
     header_.compression_algorithm = *algorithm;
+    header_.op_count_max = options_.op_count_max;
 
     if (parts.size() > 1) {
         if (!android::base::ParseUint(parts[1], &compression_.compression_level)) {
@@ -163,7 +164,7 @@
             return false;
         }
     }
-    header_.op_count_max = options_.op_count_max;
+
     resume_points_ = std::make_shared<std::vector<ResumePoint>>();
 
     if (!Sync()) {