libafs: ensure one nat ping connection per srvAddr

libafs: ensure one nat ping connection per srvAddr

track the natping conn with the srvAddr, ensuring exactly one.

Reviewed-on: http://gerrit.openafs.org/6706
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 2378895fc66a19a050f302711f2e18dbbf2e3d6f)

Change-Id: I5e74ec3f46f9af335653b6910d2c31c788181c5c
Reviewed-on: http://gerrit.openafs.org/6772
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>