interface-ip: fix race condition in IPv6 prefix address generation

interface-ip: fix race condition in IPv6 prefix address generation

Don't generate an IPv6 prefix address without taking into account the
interface state. In case eui64 is configured to generate the ifaceid this
could fail as the layer3 device mac address could not yet be available if
the interface is not yet in setup or up state.
While at it remove the interface metric assignment as this is already done
by the function interface_set_route_info.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>