Add garbage collection for dead, revoked and expired keys.  This involved
erasing all links to such keys from keyrings that point to them.  At that
point, the key will be deleted in the normal manner.

Keyrings from which garbage collection occurs are shrunk and their quota
consumption reduced as appropriate.

Dead keys (for which the key type has been removed) will be garbage collected
immediately.

Revoked and expired keys will hang around for a number of seconds, as set in
/proc/sys/kernel/keys/gc_delay before being automatically removed.  The default
is 5 minutes.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>