[IPSEC] xfrm: Use IPPROTO_MAX instead of 256

The size of the type_map array (256) comes from the number of IP protocols,
i.e., IPPROTO_MAX.  This patch is based on a suggestion from Ingo Oeser.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>