[ALPS05560355] IPv6: add <iface>/ra_mtu proc

When the MTU option is carried in the RA message, the value
will be stored in the "proc/sys/net/ipv6/conf/<iface>/ra_mtu".
Meanwhile, ra_mtu is a standalone proc, which is unaffected
by updates to the interfaces mtu value.

MTK-Commit-Id: 49f555ba3ca9d42e13baeb7833a903d765fcd490

Change-Id: Id9821fca238c79964886c7181ea1422ad91fe019
Signed-off-by: Rocco Yue <rocco.yue@mediatek.com>
CR-Id: ALPS05560355
Feature: IPv6 over MD