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.

Reviewed-on: http://gerrit.openafs.org/6419
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 79f69f2eb130c1c5cb6cdfa4e7a129f450a58339)

Change-Id: I482ad6a1fef3896144855b58c70e73bdf896027f
Reviewed-on: http://gerrit.openafs.org/7942
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>