LU-2304 lov: check enq flags when matching top lock

LU-2304 lov: check enq flags when matching top lock

cld_enq_flags must be checked when matching top lock, otherwise
if a sublock is missing then it will be reenqueued with wrong enq
flags.

For LU-2304, a previous truncate to zero lock(with CEF_DISCARD_DATA)
flags was matched by not-to-zero truncate and then wrongly reenqueued
with discard flag. This caused dirty pages to be discarded on the
other clients.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I290df2fdb9453c2fd7c8c3d3e7289e60bae9b095
Reviewed-on: http://review.whamcloud.com/4651
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>