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.

Change-Id: I4022a914bbaa0e1f3f4daadfdc32d165a6e2febd
Reviewed-on: http://gerrit.openafs.org/6418
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>