LU-10048 ofd: take local locks within transaction

LU-10048 ofd: take local locks within transaction

The patch (with companion patch LU-10048 osd: async truncate)
addresses long outstanding technical debt resulting in different
locking order on MDT and OST. With OUT introduction this mismatch
lead to deadlocks as OUT is used by the both sides and can't easily
support two different locking models simultanously.

The unified locking rules are:
- open transaction (dt_trans_start()),
- then take a lock (dt_{read|write}_lock()).

Change-Id: I1eaeb58ae3d61869914293a9fea2d0a1faefe76b
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/31293
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>