Avoid panic on shutdown with memcache and INVARIANTS

Avoid panic on shutdown with memcache and INVARIANTS

When running with memcache, the filesystem cache is not initialized,
so the file-local struct vnode *volumeVnode is never set to non-NULL.
However, shutdown_cache() unconditionally calls vrele(volumeVnode),
which triggers a KASSERT that the argument to vrele() is non-NULL.
For certain kernel configurations, this results in a panic.
Change this so that vrele() is only conditionally called if the
volumeVnode is non-NULL.

Change-Id: I80235f0d45ae11f26e9a09854bb34439f89bbb40
Reviewed-on: http://gerrit.openafs.org/1576
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>