afs: Add some comments on GetValidDSlot panics

A couple of call sites for afs_GetValidDSlot currently panic if an
error is returned, but no explanation is given. Add a few comments
helping explain why there is a panic there, instead of graceful error
handling.

Reviewed-on: http://gerrit.openafs.org/8407
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 3081a2cff2e4f9499268854504873fa801d50ce5)

Change-Id: I3bbe8f32683cb6086add494641b206c5b861fe3a
Reviewed-on: http://gerrit.openafs.org/10358
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>