afs: Cope with afs_GetValidDSlot errors

afs: Cope with afs_GetValidDSlot errors

Make callers of afs_GetValidDSlot deal with getting a NULL dcache,
which can occur if an error is encountered. Some of these just panic
at least for now, since a code path for recovery is complex, but this
is at least better than dereferencing a NULL pointer.

Reviewed-on: http://gerrit.openafs.org/6418
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 9ed26da26f4a5a3fef3bf0a7b6f9dae751ce6659)

Change-Id: I79c6fb3ae6279b5da482f95b4d4ed457beeaf1dd
Reviewed-on: http://gerrit.openafs.org/7941
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>