The 'lock_fd' is the same as 'lk->fd'. No need to store it twice so remove
it.
No functional changes intended.
Signed-off-by: Stefan Beller <sbeller@google.com> Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>