LU-4423 ptlrpc: incorporate BUILD_BUG_ON into ptlrpc_req_async_args()

LU-4423 ptlrpc: incorporate BUILD_BUG_ON into ptlrpc_req_async_args()

Every call to ptlrpc_req_async_args() should be preceded by a
BUILD_BUG_ON() (aka LASSERT()), though a few aren't.

To improve maintainability, include the BUILD_BUG_ON into the
ptlrpc_req_async_args() macro.

Change-Id: I07481921379930b4b2d9329aefb47068fd0e07f0
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35509
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>