afs_AllocDCache: return NULL instead of panic

Currently afs_AllocDCache will panic if we cannot get a valid dcache
from the free/discard lists. Instead, return NULL, so the caller can
decide how to handle the error.

Currently the caller will just panic anyway, but that will be
addressed in a future commit.

Reviewed-on: http://gerrit.openafs.org/8377
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 64ee7b32f134e6431eb2894ed4397f8b1fea25ad)

Change-Id: Ica8f1539279c30f4914ab84161c6bc2f0468d565
Reviewed-on: http://gerrit.openafs.org/10355
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>