avoid downward vcache pressure when entries are free

avoid downward vcache pressure when entries are free

we try to keep VCACHE_FREE entries free. if there's already that many free,
do nothing.

Reviewed-on: http://gerrit.openafs.org/4595
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit fd545c51fe34b1452f09d4fbf6458f5b1557e895)

Change-Id: Ia19b748596eadc75c6487d5fe854d2c7c4a4595a
Reviewed-on: http://gerrit.openafs.org/4615
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>