librbd: fix refuse to release lock when cookie is the same at rewatch

librbd: fix refuse to release lock when cookie is the same at rewatch

  fix exclusive auto-mode lock refuse to release.
  when rewatch, owner_id is reset.
  at the same time, there is a chance to produce the same cookie, which should be different.
  code now skips reacquire lock when the cookie is the same, resulting in unsetting owner_id.
  when other clients request lock, client whose owner_id is null is considered invalid and
  refuse to release lock.
  but unluckily, watcher is always alive, so the client requested lock can't get lock.

Signed-off-by: Song Shun <song.shun3@zte.com.cn>