logger: unlock the mutex for fsync & call fsync only when backend is running

- fsync can be a long blocking operation, so we need to make sure the main
  logger thread does not block during this time, when it tries to aquire
  the mutex
- fixes calling fsync on an invalid file descriptor

src/modules/logger/log_writer_file.cpp

@@ -257,8 +257,10 @@ void LogWriterFile::run()
 						buffer.close_file();
 					}
 
-				} else if (call_fsync) {
+				} else if (call_fsync && buffer._should_run) {
+					pthread_mutex_unlock(&_mtx);
 					buffer.fsync();
+					pthread_mutex_lock(&_mtx);
 
 				} else if (available == 0 && !buffer._should_run) {
 					buffer.close_file();