exempt instant timeouts from mtu discovery

exempt instant timeouts from mtu discovery

if we set lastReceiveTime to 0 to hint that no net, honor that here as
"just time out"

Reviewed-on: http://gerrit.openafs.org/2875
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 0ecc90f3cd3f5a7c96b3689ab0c5f6a42edc3ed0)
Change-Id: Ic85f7ea234a50935c5fad9804d459b6198902e4c
Reviewed-on: http://gerrit.openafs.org/3038
Tested-by: Derrick Brashear <shadow@dementia.org>