fatal-signal: Fix clang error due to lock.

fatal-signal: Fix clang error due to lock.

Due to not acquiring lock, clang reports:
  lib/vlog.c:618:12: error: reading variable 'log_fd' requires holding mutex
  'log_file_mutex' [-Werror,-Wthread-safety-analysis]
  return log_fd;

The patch fixes it by creating a function in vlog.c to write
directly to log file unsafely.

Tested-at: https://travis-ci.org/github/williamtu/ovs-travis/builds/666165883
Fixes: ecd4a8fcdff2 ("fatal-signal: Log backtrace when no monitor daemon.")
Suggested-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: William Tu <u9012063@gmail.com>