LU-16044 osd: discard pagecache in truncate's declaration

LU-16044 osd: discard pagecache in truncate's declaration

to avoid taking pagelock inside a transaction which conflicts
with the write path where we take pagelock before any another one.
this should be safe as the write path writes the pages out
synchronously, so they should be clean by truncate.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: Iba555ace2ce9ef34ab5517375ecb5c176f738a02
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48033
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>