staging/lustre/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.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-on: http://review.whamcloud.com/8806
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-181


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>