libafs: initialize free dcache list for memcache

libafs: initialize free dcache list for memcache

reuse the disk cache dcache creation code for memcache, then
call it to prime the list

Reviewed-on: http://gerrit.openafs.org/7446
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 9bb876f6829f137e534b961f7374b310bfa8aa7d)

Change-Id: I1c616521a9e518ac7e4e62006b3a327458263cae
Reviewed-on: http://gerrit.openafs.org/7944
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>