netfilter: nft_tunnel: add missing attribute validation for tunnels

commit 88a637719a1570705c02cacb3297af164b1714e7 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>