afs: Add afs_WriteDCache sanity checks

afs: Add afs_WriteDCache sanity checks

Writing a non-free non-discarded dcache entry with a zero volume id
can easily cause hash table corruption later on, so make sure we don't
do that. Also log something if the write itself fails, as this usually
indicates an unusual situation involving I/O errors or something.

Change-Id: Ib9602227e8cee324cb63a4a3dee28e53af69b446
Reviewed-on: http://gerrit.openafs.org/6419
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>