LU-10048 osd: async truncate

LU-10048 osd: async truncate

osd-ldiskfs should execute truncate outside of main transaction
handle. This avoids restarting truncate transaction handles in
main transaction, and allows "transaction first, locking second"
model on OST.

Change-Id: Iffe45c42834c26ca72b65e068ad25ac61d0607c8
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: https://review.whamcloud.com/27488
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>