LU-18813 osd-wbcfs: make dt_last_seq_get() optional

LU-18813 osd-wbcfs: make dt_last_seq_get() optional

Allow osd-wbcfs to leave this unimplemented for
now. Once we track object mapping internally,
we can implement this.

Fixes: 373b76b345b5 ("LU-17658 fid: check on disk sequence before allocating to osp")
Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Iea4401db0c7656fd56c43f6e0d296cabce89e864
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59502
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>