afs: avoid panic in DNew when afs_WriteDCache fails

afs: avoid panic in DNew when afs_WriteDCache fails

afs_WriteDCache may fail for an IO error, or if interrupted (EINTR).
Unfortunately, DNew will panic in this case, crashing the entire
machine.

In order to avoid an outage in this case, don't panic.  Instead, reflect
the error back to the caller of DNew.

While here, add Doxygen comments to DNew.

Change-Id: I27a8f89bab979c5691dded70e8b9eacbe8aff4fd
Reviewed-on: https://gerrit.openafs.org/13804
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>