LU-10026 ldiskfs: mballoc to preserve preallocation's start

LU-10026 ldiskfs: mballoc to preserve preallocation's start

.. used in dense preallocation. otherwise it's possible to lose
preallocated space when the corresponding cache bitmap is dropped
from the cache, then ldiskfs will be printing error messages
about block counter mismatch.

Fixes: 686dee707f ("LU-10026 osd-ldiskfs: use preallocation for dense writes")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I93177510af959e849dba7a9c35d81bc27809a31b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55467
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>