af_key: constify lookup tables

The lookup tables for minimum sizes of extensions and for the pfkey
handler functions are read only, therefore can be const.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>