LU-874 ldlm: prioritize LDLM_CANCEL requests

LU-874 ldlm: prioritize LDLM_CANCEL requests

If a lock canceling request has already reached server, there is no
reason of evicting the client when the waiting list times out.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I1a300916b4b3b592ca565ffc06cb3658d699d7a0
Reviewed-on: http://review.whamcloud.com/1900
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>