afs: Correct routine name on error message

While studying some code I noticed one of the error messages in
afs_UFSGetVolSlot was prefixed with a different routine name.
More shocking was that git blame fingered me as the last person
to update that line!  Indeed I had but I hadn't noticed, nor had
my reviewers, the mis-matched routine name.

Update afs/afs_volume.c to correct routine name.

Change-Id: Id7ee275c9f8991bb71082b9dfcd52c14ead83955
Reviewed-on: http://gerrit.openafs.org/11625
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>