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.

Change-Id: I2ee151ec1166a78475047bffd2c1d31f45e8dbdf
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>