LU-2367 osc: unplug IO queue for writeback anyway

LU-2367 osc: unplug IO queue for writeback anyway

osc_io_unplug_async() in osc_extent_release() may not unplug IO
queue due to a race in ptlrpc_queue_work(). As a fix, we should
unplug it when the page is being flushed or fsync calls.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ied1d6ad12b79a28229cdae83a9db9ad9b3189462
Reviewed-on: http://review.whamcloud.com/4659
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>