Make netif REQUIRE hostid

As noted in the PR, if etc/rc.d/zvol is removed, netif will be run before
hostid, and the MAC address generated for any bridge devices will be
non-deterministic. Make the MAC address generated be deterministic for
bridge devices by explicitly REQUIRE'ing hostid.

This fixes up the rest of the PR, inadvertently committed in r299844

MFC after: 1 week
PR: 195188
Sponsored by: EMC / Isilon Storage Division