LU-884 osc: async osc_check_rpcs()

LU-884 osc: async osc_check_rpcs()

Add a new "async" parameter to osc_check_rpcs(); if it is called with
async, it will compose a fake ptlrpc_request so that RPCs will be
composed and issued in ptlrpcd context.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I1c2f8ae43da8146428c474f17ddf3dc23a2df9ef
Reviewed-on: http://review.whamcloud.com/1825
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>