KEYS: struct key_preparsed_payload should have two payload pointers

struct key_preparsed_payload should have two payload pointers to correspond
with those in struct key.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Sage Weil <sage@redhat.com>