LU-7830 ost: do not evict during truncate

LU-7830 ost: do not evict during truncate

OST punch request can last for a long time, so
it is needed to prolong taken lock
until it completes.

Seagate-bug-id: MRP-3332
Change-Id: Icad7d122bc559f9e0bd693c7f88cbac14235bd38
Signed-off-by: Andriy Skulysh <andriy.skulysh@seagate.com>
Reviewed-on: http://review.whamcloud.com/18739
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>