libafs: Speed up afs_CheckTokenCache

On systems with a large number of PAGs and files in use, the
periodic daemon job that checks for expired credentials and
cleans up the axs cache can run for a very long time.  This
can lead to kernel soft lockups and eventually hang processes
and file access because of unavailable locks.

Rework the scanning logic in afs_CheckTokenCache to make the
scanning more efficient in most real world cases.  On a test
system accessing ~4000 files from processes in 1000 PAGs, this
has been observed to reduce the runtime of afs_CheckTokenCache
from a problematic ~70s down to about 0.7s.

Additionally, this changes the conditions in which an axscache is
discarded. uid+cell (rather than just uid) must now match, and
if no matching unixuser is found, it will also be discarded.

Adapted from code from Jeffrey Altman who provided the original
loop algorithm and code.

Change-Id: I65b275b4244b3b6ab65453623bb8729530a9e1a6
Reviewed-on: http://gerrit.openafs.org/11123
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>