afs: WriteThroughDSlots: Avoid write error panic

afs: WriteThroughDSlots: Avoid write error panic

Currently, afs_WriteThroughDSlots panics if our call to
afs_WriteDCache fails. Since afs_WriteThroughDSlots is called every
minute by a background daemon, this means that if our cache fs becomes
inaccessible (by being forced read-only, or for any other reason), we
are virtually guaranteed to panic relatively quickly.

To try to avoid this at least for some cases, change
afs_WriteThroughDSlots to return an error to our caller when we
encounter such an error. For our background task, we can just ignore
the error and retry the writes on a future iteration. During shutdown,
we still panic if we encounter an error, to try to avoid silently
allowing a corrupt cache to be used on subsequent boots.

Change-Id: Ia5f180a5c709881c3e884629c02e9ff93729fa88
Reviewed-on: https://gerrit.openafs.org/13047
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>