radvdump fix to interpret MTU and Route

MTU was simply skipped and the following route Information
::/0 with length 1 was not interpreted correctly, the address
printed out begin with the bytes for the next option (the
next route).  The route information was always length 2, this
is not correct according to RFC 4191 2.3. Route Information Option

Signed-off-by: Reuben <reubenhwk@gmail.com>