Forcibly remove the cached items from pseudofs vncache on module unload.

If some process' nodes were accessed using procfs and the process
cannot exit properly at the time modunload event is reported to the
pseudofs-backed filesystem, the assertion in pfs_vncache_unload() is
triggered. Assertion is correct, the cache should be cleaned.

Approved by: des (pseudofs maintainer)
Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week