LU-9033 llite: don't zero timestamps internally

LU-9033 llite: don't zero timestamps internally

In ll_md_blocking_ast(), we zero all timestamps to avoid these
'leftovers' interfering the new timestamps from MDS, especially
when the timestamps are set back by other clients. It's not
quite right to change timestamps in this way, because:

1. The pending lock can be matched by getattr, so these zero
   timestamps can be fetched by application in a small race window.

2. It doesn't make sense to zero the mtime and ctime, because we
   always use the newest ctime and mtime from MDS when do attributes
   merge, they won't interfere new timestamps set by other clients.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Ieb9577abe4938bc47dc0577454a4a1bbf4796876
Reviewed-on: https://review.whamcloud.com/24984
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>