[netdevice] Make ll_broadcast per-netdevice rather than per-ll_protocol

[netdevice] Make ll_broadcast per-netdevice rather than per-ll_protocol

IPoIB has a link-layer broadcast address that varies according to the
partition key.  We currently go through several contortions to pretend
that the link-layer address is a fixed constant; by making the
broadcast address a property of the network device rather than the
link-layer protocol it will be possible to simplify IPoIB's broadcast
handling.