KEYS: user_defined: sanitize key payloads

Zero the payloads of user and logon keys before freeing them.  This
prevents sensitive key material from being kept around in the slab
caches after a key is released.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>