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.

This is revive of git commit 83ae17df2bdce837e62473aec27c03d67312c8ea.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I2ac32866f7adbc4701370704612c849a18a5d1ac
Reviewed-on: http://review.whamcloud.com/10292
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>