crypto: allocate padding in crypto_header

while it might seem a waste of space by using 2 extra bytes in
the crypto_config_header, it actually gives us the option
to grow "unknown at this time" features without hopefully
breaking onwire compat

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>