LU-181 obdclass: fix portal_handle memory wastage

LU-181 obdclass: fix portal_handle memory wastage

This patch is the first step of fixing memory wastage in Lustre.
We can save 20 bytes for each portals_handle(on 64bits system)
by reusing members, which could be over a hundred megabytes
on system with millions of ldlm_locks because portals_handle is
embedded in ldlm_lock.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: I401c0c0c2fc8d2624fd48c714dc3d06fc0e4e21e
Reviewed-on: http://review.whamcloud.com/1827
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>