SOLARIS: Free vcache mappings on shutdown

SOLARIS: Free vcache mappings on shutdown

Right before shutdown, go through all of our vcaches and flush them,
freeing any pages associated with the vcaches. If we don't do this,
pages associated with our vcaches may still be around after we
shutdown and the module is unloaded, causing a panic when the kernel
tries to deference the page's vnode.

Abstract out afs_freevfs() following the gafs_foo/afs_foo convention
from the vnode ops, since we're calling this in two different places.

Change-Id: I64b88799acce3a55648ec94b53ed63319cd04698
Reviewed-on: http://gerrit.openafs.org/3472
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>