KEYS: Call ->free_preparse() even after ->preparse() returns an error

Call the ->free_preparse() key type op even after ->preparse() returns an
error as it does cleaning up type stuff.

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>