LU-9997 ldlm: check lock cancellation in ldlm_cli_cancel()

LU-9997 ldlm: check lock cancellation in ldlm_cli_cancel()

In that case, the assert for 'list_empty(&lock->l_bl_ast)' will fail
because the lock is already in a cancel list.

This patch checks if the lock is already being canceled in prior.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I04b9df789cf3b3b5e6dd61a647376307f9975f4a
Reviewed-on: https://review.whamcloud.com/29080
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>