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.

Change-Id: If4b88cbc7f9af6343353648837b28541a5088429
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>