afs: prevent nat pings on destroyed connections

afs: prevent nat pings on destroyed connections

If the connection is no longer in use, reset the nat keep alive
timer to 0 seconds.

Reviewed-on: http://gerrit.openafs.org/6230
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 7a93c827f353e2ad695d8019ab789b02a3596f2e)

Change-Id: I019bdce7a8f9f0123786b46d0563f42f7b26c14b
Reviewed-on: http://gerrit.openafs.org/6232
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>