LU-11760 ofd: limit num of objects to create in 1 transaction

LU-11760 ofd: limit num of objects to create in 1 transaction

Set flag th_sync when the number of objects to create per
sequence reaches OST_MAX_PRECREATE in one transaction.
It is needed to avoid gaps after OST failover.
See details in LU-11760.

Lustre-change: https://review.whamcloud.com/35373
Lustre-commit: 4485ee8be4cf224e2543f6344efc6e1cb295a0a7

Change-Id: Ie29de5a42e757b07561749982359c01df999e798
Signed-off-by: Sergey Cheremencev <c17829@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35951
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>