Only a handful of xfrm_types exist, no need to have 512 pointers for them.

Reduces size of afinfo struct from 4k to 120 bytes on 64bit platforms.

Also, the unregister function doesn't need to return an error, no single
caller does anything useful with it.

Just place a WARN_ON() where needed instead.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>