LU-5781 ldlm: Solve a race for LRU lock cancel

LU-5781 ldlm: Solve a race for LRU lock cancel

This patch solves a race condition that the lock may be used again
after LRU cancellation policy check. In that case, the lock may have
locked or dirty pages that makes the policy check totally useless.
The problem is solved by checking l_last_used at cancellation time
therefore it can make sure that the lock has not been used.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Change-Id: I3c7e2a3cc84fa9f549cf002e751018c022493606
Reviewed-on: http://review.whamcloud.com/12603
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>