filelock.Unlock() was called twice for fcntl implementation if an error
occurs during file.{,R}Lock(): once in the error handler, once in
filelock.lock().

Change-Id: I5ad84e8ef6b5e51d79e0a7a0c51f465276cd0c57
Reviewed-on: https://go-review.googlesource.com/c/152717
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>