afs_AllocDCache: return NULL instead of panic

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.

Change-Id: Iafb539bbda9ef20907ef575699185f111ca39c2b
Reviewed-on: http://gerrit.openafs.org/8377
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>