LU-181 ptlrpc: align fields of ptlrpc_request

LU-181 ptlrpc: align fields of ptlrpc_request

Size of ptlrpc_request_buffer_desc is 1048 which means we need 2K
slab for allocating rqbd. This can be changed after we align
fields in ptlrpc_reques to decrease size of ptlrpc_requestt,
because each rqbd has an embedded ptlrpc_request.
We also aligned fields of lu_context because it's embedded in
ptlrpc_request.

With this simple change, size of rqbd is 1008 bytes, and we
can save at least tens of MBs on server.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: Iac292e4375421843785fdae083b1aa667d43b3a7
Reviewed-on: http://review.whamcloud.com/3352
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>