fix redundant flushes of partfile metadata

ChangeLog

@@ -1,3 +1,4 @@
+	* fix redundant flushes of partfile metadata
 	* add option to ignore min-interval from trackers on force-reannounce
 	* raise default setting for active_limit
 	* fall back to copy+remove if rename_file fails

src/part_file.cpp

@@ -418,5 +418,6 @@ namespace libtorrent
 		file::iovec_t b = {header.get(), size_t(m_header_size) };
 		m_file->writev(0, &b, 1, ec);
 		if (ec) return;
+		m_dirty_metadata = false;
 	}
 }