LU-13368 lnet: discard the callback

LU-13368 lnet: discard the callback

Lustre need a completion callback for event that request
has been sent. And then need other callback when reply
arrived. Sometime the request completion callback maybe
lost by some reason even reply has been received.
system will wait forever even timeout. We needn't to wait
request completion in such case. So provide a way to
discard the callback.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: If9cd8420ee76947ee5053180e0f5219f76bb94c2
Reviewed-on: https://review.whamcloud.com/38845
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>