afs: Avoid afs_GetDCache panic on cache open error

afs: Avoid afs_GetDCache panic on cache open error

When we need to populate a dcache entry, afs_GetDCache calls
afs_CFileOpen to get a handle for our file backing that dcache.
Currently, if we cannot open the file, we panic.

To handle this a little more gracefully, just return an error from
afs_GetDCache instead. The relevant userspace request will probably
fail with EIO, but this is better than possibly crashing the whole
system.

Reviewed-on: https://gerrit.openafs.org/13046
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 22e64df8e043fa7bd78bff263866ee2bd6a6e13d)

Change-Id: Iddc7b0ebc56a64d792251f1f42c576603f2a86cf
Reviewed-on: https://gerrit.openafs.org/13072
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>