[XFRM]: Cleanup xfrm_msg_min and xfrm_dispatch

Converts xfrm_msg_min and xfrm_dispatch to use c99 designated
initializers to make greping a little bit easier. Also replaces
two hardcoded message type with meaningful names.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>