LU-3188 osc: shorten IO calling path

By using osc_io_unplug_aync() for osc_queue_sync_pages() to shorten
the IO calling path, to reduce the chance of stack overflow.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I06112d9fb7b069d68c7945641e65d6430d0bb380
Reviewed-on: http://review.whamcloud.com/6191
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Keith Mannthey <kemannthey@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>