LU-16966 osd: take trunc_lock for fallocate

LU-16966 osd: take trunc_lock for fallocate

as fallocate may need few transactions (or transaction restarted)
we have to avoid any concurrent writes/truncates on this object
until fallocate supports 'restart-from-beginning' - first stop the
transaction, then release the lock, then repeat again (like
the write path does).

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I0bf38b1886fbf24656b45fe0f87fcbad2227672a
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52264
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>