netfilter: nft_tunnel: add missing attribute validation for tunnels

Add missing attribute validation for tunnel source and
destination ports to the netlink policy.

Fixes: af308b94 ("netfilter: nf_tables: add tunnel support")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>