Don't count root session keyrings against quota

Don't count root session keyrings against quota

AFS PAM modules can call setpag() as root, regardless of the UID
being authenticated. This leads to the session keyring being created
using roots quota - on some systems (RHEL5) this quota is both small,
and of a fixed size.

Modify our keyring allocation code so that when a keyring is created
by root, we don't do any quota checks.

Reported-by: Rainer Toebbicke <rtb@pclella.cern.ch>
Reviewed-on: http://gerrit.openafs.org/1577
Tested-by: Rainer Toebbicke <rtb@pclella.cern.ch>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit c4537f0442ac7ecbf8c946de45004992e17d535f)

Change-Id: Ia152db35b756abc3b3684194ea3c155f22fef537
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
(cherry picked from commit 9bd2fe688f3d1cea48a7d261db3bceed6a244a85)