LU-12509 ofd: ofd_precreate_objects lockdep warning

LU-12509 ofd: ofd_precreate_objects lockdep warning

ofd_precreate_objects()->ofd_write_lock() is used to
get the write locks, later dt_write_lock() is called
to do the same on the last_id object.

This triggers a false positive with lockdep enabled,
use a different sublass on the last_id to silence
the warning.

Consolidate lockdep subclasses into a single place,
dt_object_role.

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I283782352bef8422e6c327ce5cfac306da289e37
Reviewed-on: https://review.whamcloud.com/35420
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>