xfrm: input: constify xfrm_input_afinfo

Nothing writes to these structures (the module owner was not used).

While at it, size xfrm_input_afinfo[] by the highest existing xfrm family
(INET6), not AF_MAX.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>