afs: more cache truncation stats

afs: more cache truncation stats

Add counters for cache too full and waiting to drain occurrences. These
will be used in later commits to indicate how often the cache truncation
is required and how often the cache manager is waiting for cache
truncation to complete.

Change-Id: I4aa802729f0910dff1fb3e90b2d44d36df8bf8f3
Reviewed-on: https://gerrit.openafs.org/13168
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>