LU-1876 hsm: account for active IOs correctly

We used to use refcount of lsm to account for active IOs but this
turns out wrong because empty layout files don't have a lsm.

In this patch, lov_object::lo_active_ios is invented to account for
active IOs for both raid0 and empty layout;

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I1b7aa16367af1d0a54f6c70bb4155b20071af225
Reviewed-on: http://review.whamcloud.com/5157
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>