staging/lustre/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>
Reviewed-on: http://review.whamcloud.com/12603
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5781


Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>