rx: avoid nat ping until connection is attached

rx: avoid nat ping until connection is attached

drop nat pings on connections we haven't talked on yet

Reviewed-on: http://gerrit.openafs.org/5130
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 496fb87372555f6acddd4fd88b03c94c85f48511)

Change-Id: Id64a771f9cb50191a665f5ccb98ec66d991f47bb
Reviewed-on: http://gerrit.openafs.org/5225
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>