LU-1527 clio: check if lock is freed in cl_lock_peek()

LU-1527 clio: check if lock is freed in cl_lock_peek()

The lock may have been freed between cl_lock_lookup() and
cl_lock_mutex_get() so we should check lock state after grabbing
lock mutex.

Xyratex-bug-id: MRP-665
Change-Id: Id3562b3dd8bd052b74ad7840f08b904ca38a6746
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Reviewed-on: http://review.whamcloud.com/3117
Reviewed-by: Jinshan Xiong <jinshan.xiong@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>