LINUX: Avoid unnecessary afs_ShakeLooseVCaches

LINUX: Avoid unnecessary afs_ShakeLooseVCaches

Before some of the NewVCache refactoring, we only called
afs_ShakeLooseVCaches when afs_vcount was over (or equal to)
afs_maxvcount. Do this again, so we only try to shake loose vcaches
when we need more vcaches. Otherwise, we will call
afs_ShakeLooseVCaches every single time we want a new vcache when
dynamic vcaches are disabled (such as 2.4 Linux), and we log a warning
when we can't free any (which is likely when there are only 1 or 2
vcaches in use).

FIXES 128756

Change-Id: Ia06f76a6f719e69881b91fb2d975cfc33c85c370
Reviewed-on: http://gerrit.openafs.org/3569
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>