afs: Handle easy GetValidDSlot errors

Many callers of GetValidDSlot currently assume they will always get
back a valid dcache, and will panic on getting NULL. However, for many
of these callers, handling the NULL case is quite easy, since the
failure to get a dcache can just result in an error directly, or
obtaining the dcache is best-effort or just an optimization.

This commit just handles the "easy" cases; some other callers require
more complex handling.

Reviewed-on: http://gerrit.openafs.org/8375
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit f74a0a7bbb37a8ab6050e833cf8d66abdff31854)

Change-Id: Ic2c463edebcb821562541004bd4181483c90c3e6
Reviewed-on: http://gerrit.openafs.org/9287
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>