afs: Log slot for GetDSlot read errors

afs: Log slot for GetDSlot read errors

When we log that a disk read error occurred during GetDSlot, log which
slot we were trying to read for convenience.

Reviewed-on: http://gerrit.openafs.org/8368
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit b5d9e29e89c2a63c857c47f1fb9accae90b0a3aa)

Change-Id: I0d5fffd3ef3af5d788db082e10c401afc0dd440a
Reviewed-on: http://gerrit.openafs.org/8544
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>