afs: Log pid with disk cache read errors

afs: Log pid with disk cache read errors

Log the current pid (and procname) when we complain about an error
when reading from CacheItems in afs_UFSGetDSlot. These errors can
result in confusing situations, so it can be helpful to know at least
what process saw the error.

Our logic for logging this information is getting a bit large, so also
move this to a new function, LogCacheError.

Change-Id: I3427e736458784df0d516f4182684605e930e128
Reviewed-on: https://gerrit.openafs.org/14416
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>