afs: Handle afs_AllocDCache errors

Do not panic if afs_AllocDCache encounters an error and returns NULL.
Instead, go into the normal retry loop that occurs if we couldn't free
up any more free/discard dcache entries.

Reviewed-on: http://gerrit.openafs.org/8405
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit e02185547e316667989a100f77e68b918481b597)

Change-Id: I507a6329b7b4fadaa39d0dc907c15fb5647fc135
Reviewed-on: http://gerrit.openafs.org/10356
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>