afs: don't try GetDownD if nothing to get

afs: don't try GetDownD if nothing to get

if we "need" negative slots or caches, don't run. also,
don't stay "too full"

Reviewed-on: http://gerrit.openafs.org/5612
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 488c7c97854a4bd0ec67bcfe17df93b3fd025f88)

Change-Id: I1bdd37ce5659ce770f0fbb23bcb9174858b5eeed
Reviewed-on: http://gerrit.openafs.org/5709
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>