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.

Change-Id: If1c259bd22a382ff56ed29326aa20c86389d06bc
Reviewed-on: https://gerrit.openafs.org/13410
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>