LU-1318 ldlm: allow different LVB structures to be used

LU-1318 ldlm: allow different LVB structures to be used

The DLM code always assumes that the request buffer storing the LVB is
of type ost_lvb.
This patch allows new LVB types to be defined and used. Although there
is still one single LVB structure defined (i.e. ost_lvb), the new
quota architecture will define its own LVB soon.

Signed-off-by: Johann Lombardi <johann@whamcloud.com>
Change-Id: I497d774eb4b66029ff2755739bbe75190f6aeee8
Reviewed-on: http://review.whamcloud.com/2649
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>