libafs: only do pings for default conn with root uid

libafs: only do pings for default conn with root uid

instead of doing it for potentially every unauth user, just do it for
root.

Reviewed-on: http://gerrit.openafs.org/6255
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 78885611ac8aa6602a4a1f42379c9d78ef226100)

Change-Id: Id54f6608b8807289242d094f48e394f0341782da
Reviewed-on: http://gerrit.openafs.org/6326
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>