Linux 3.8: session_keyring changes

The session_keyring is now attached directly to the cred structure
and the thread_group_cred structure (cred->tgcred) no longer exists.

Adapt code that makes use of tgcred, and use the standard rcu based
mechanism to update session_keyring.

Reviewed-on: http://gerrit.openafs.org/8905
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit c6d27e322a8d3b352554650473a048235a9c763a)

Change-Id: If0998ee777ab0d196e19162499eb3ee60c5ff85d
Reviewed-on: http://gerrit.openafs.org/8941
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>