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.

Lustre-change: https://review.whamcloud.com/48033
Lustre-commit: 0bb491b2ecf494c3f78fa08a101af8af7853a0fe

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/ex/lustre-release/+/48885
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>