Openwrt pppoe MTU issue

OpenWrt will get pppoe mtu from wan interface mtu option in the uci,
but if you set mtu in the wan interface, the physical interface's mtu will change, too.

And the pppoe header's length is 8 bytes, if you set mtu to 1500, will get 1500-8=1492 pppoe mtu.

 This is wrong, so you need to change the mtu parameter that ppp daemon parsing in the uci.
You may add a new option in uci something like pppoe_mtu, and modified the ppp script that parsing uci.