LU-9543 ofd: fiemap deadlock

LU-9543 ofd: fiemap deadlock

lock_zero_regions() locks all zero regions by acquiring
a set of independent locks.
It can deadlock with a PW lock for the whole file from
a client.

Indeed it isn't required to have all zero regions locked
at once, we need only force clients to flush data for
these regions.

Change-Id: Ib48e2bd9e6f715eb54a7821acde7b38b0de6650c
Seagate-bug-id: MRP-4393
Signed-off-by: Andriy Skulysh <andriy.skulysh@seagate.com>
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/15734
Reviewed-by: Artem Blagodarenko <artem.blagodarenko@seagate.com>
Reviewed-by: Vitaly Fertman <vitaly.fertman@seagate.com>
Tested-by: Elena V. Gryaznova <elena.gryaznova@seagate.com>
Reviewed-on: https://review.whamcloud.com/27224
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Artem Blagodarenko <c17828@cray.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>