LU-15117 ofd: no lock for dt_bufs_get() in read path

LU-15117 ofd: no lock for dt_bufs_get() in read path

osd_bufs_get() allocates the pages and can cause new transactions
as part of memory release procedure. this would break Lustre's
"start a transaction, then do locking" rule.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I782f0cc6c96251ad88d5fb8d15c9ac91d382bf7e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48209
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>