LU-962 ptlrpc: feature to run callback in ptlrpcd context

LU-962 ptlrpc: feature to run callback in ptlrpcd context

In this patch, a feature is added to run a callback in ptlrpc
context. We need a ptlrpc work for this purpose. There are three
functions exported:
  1. ptlrpc_alloc_work() to allocate work;
  2. ptlrpc_run_work() to run an allocated work, this function can
     be executed many times;
  3. ptlrpc_destroy_work() to destroy the work;

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I2bce5a17003855468eab9075fb50ed02d7bcc208
Reviewed-on: http://review.whamcloud.com/1917
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>