The encap->type comes straight from Netlink. Validate it against
max supported encap types just like ip_encap_hlen() already does.

Fixes: a8c5f9 ("ip_tunnel: Ops registration for secondary encap (fou, gue)")
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>