LU-2970 lov: wait for active IOs before deleting the object

LU-2970 lov: wait for active IOs before deleting the object

lov_layout_operations::llo_delete will be called when the file's
layout is going to change or file is being taken out of cache so we
must make sure there is no active IOs on this object.

For layout change case, it's lov_layout_change()'s responsibility to
make sure there is no active IOs before entering into llo_delete.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Id56a66548f9f05555e6d112355d2a71da5ab4705
Reviewed-on: http://review.whamcloud.com/5812
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>