netifd: inherit ip6 tunnel tos value

In case of ipip6 tunnels, inherit the tos value in the ip6 header from the inner ip4 header as the default behaviour. Without this commit, the tos value in the ip6 header is hard coded to 0 which is wrong.

The ipip6 mode tunnels lack a tos uci option allowing the choice of either inheriting tos value or setting it as per uci config. I will push a change to address this separately, but, in the absense of such a config, the default behaviour should be inherit the tos value from the inner ip4 header and not to hard code it to 0. This commit fixes that.