LU-10186 osd-zfs: move LAST_ID OI mapping out of oi.xx

LU-10186 osd-zfs: move LAST_ID OI mapping out of oi.xx

Originally, the OI mapping for LAST_ID was put under the
oi.xx file, it may cause some issues:

1) If we remove the oi.xx file, then before the OI scrub
   scanned the whole device, we can not know the last ID
   for related sequence. That will fail the system start
   as to the OI scrub will be stopped by force.

2) It is incompatible with ldiskfs backend, that will cause
   trouble after migrating from ldiskfs-based device.

This patch moves the LAST_ID OI mapping from oi.xx file to
/O/<seq>/LAST_ID as ldiskfs backend does.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ibfef1d9ef5be72f6e04ff79f92302483f7fe8a15
Reviewed-on: https://review.whamcloud.com/28679
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>