bridge: tunnel: fix attribute checks in br_parse_vlan_tunnel_info

These checks should go after the attributes have been parsed otherwise
we're using tb uninitialized.

Fixes: efa5356b ("bridge: per vlan dst_metadata netlink support")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>