LU-10438 flr: layout truncate compatibility

LU-10438 flr: layout truncate compatibility

In PFL design, client issues [0, size) intent to MDS to instantiate
objects in this extent. While in FLR design, the intent serves two
purposes: 1) make objects across [size, EOF) on other mirrors stale,
and 2) instantiate objects in the chosen write mirror. And original
FLR chose to use [size, EOF) as the extent of truncate write intent
request.

This patch reverts the choice, and still uses [0, size) as the
truncate write intent extent.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I12d90320ba704f01457670f864d78fe764233000
Reviewed-on: https://review.whamcloud.com/30786
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>