[era_restore] Avoid touching the output file by checking input file earlier

The output file has been checked by the caller, so there's no need
to check the output file again.

era/era_restore.cc

@@ -37,7 +37,7 @@ namespace {
 		bool metadata_touched = false;
 		try {
 			block_manager::ptr bm = open_bm(*fs.output, block_manager::READ_WRITE);
-			file_utils::check_file_exists(*fs.output, false);
+			file_utils::check_file_exists(*fs.input);
 			metadata_touched = true;
 			metadata::ptr md(new metadata(bm, metadata::CREATE));
 			emitter::ptr restorer = create_restore_emitter(*md);