Allow keys for which the key type has been removed to be unlinked.  Currently
dead-type keys can only be disposed of by completely clearing the keyrings
that point to them.
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>