LU-2645 ldlm: use correct lvb size to reply 1.8 lock enqueue

For 1.8 client, it does not support variable-sized LVB.
The 2.4 server should correctly distinguish whether the
client support it or not, and fill the reply buffer with
suitable LVB size when processing lock enqueue.

Test-Parameters: envdefinitions=SLOW=yes,ENABLE_QUOTA=yes \
clientjob=lustre-b1_8 clientbuildno=256 testlist=runtests

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I9241efe25dc64b26e86c4e75da72ab74bb1bc750
Reviewed-on: http://review.whamcloud.com/5459
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>