LU-181 ptlrpc: reorganize ptlrpc_request

LU-181 ptlrpc: reorganize ptlrpc_request

ptlrpc_request has some structure members are only for client side,
and some others are only for server side, this patch moved these
members to different structure then putting into an union.

By doing this, size of ptlrpc_request is decreased about 300 bytes,
besides saving memory, it also can reduce memory footprint while
processing.

Another change in this patch is, osp will not use rq_exp_list anymore
because it is a server only member now.
osp will use ptlrpc_req_async_args to store commit_cb parameters in
this patch.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Change-Id: Id910ac225b8e9d33a0cae40b3124ce55f1a3fbc9
Reviewed-on: http://review.whamcloud.com/8806
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>