afs: Reword "cache is full" messages

afs: Reword "cache is full" messages

Currently, there are multiple different areas in the code that log a
message that look like this, when we encounter an ENOSPC error when
writing to the cache:

    *** Cache partition is FULL - Decrease cachesize!!! ***

The message is a bit unclear, and doesn't even mention AFS at all.
Reword the message to try to explain a little more what's happening.

Also, since we log the same message in several different places, move
them all to a common function, called afs_WarnENOSPC, so we only need
to change the message in one place.

Reviewed-on: https://gerrit.openafs.org/13410
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit d9d9571785dabc5c311111b1263fe0881b0ccda5)

Change-Id: I8e958f2896f5d4503d3a153b52720e8ba3025261
Reviewed-on: https://gerrit.openafs.org/13417
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>