MFC r363244: ether_ifattach: set mtu before calling if_attach()

if_attach() -> if_attach_internal() will call if_attachdomain1(ifp) any time
an ethernet interface is setup *after*
SI_SUB_PROTO_IFATTACHDOMAIN/SI_ORDER_FIRST. This eventually leads to
nd6_ifattach() -> nd6_setmtu0() stashing off ifp->if_mtu in ndi->maxmtu
*before* ifp->if_mtu has been properly set in some scenarios, e.g., USB
ethernet adapter plugged in later on.

For interfaces that are created in early boot, we don't have this issue as
domains aren't constructed enough for them to attach and thus it gets
deferred to domainifattach at SI_SUB_PROTO_IFATTACHDOMAIN/SI_ORDER_SECOND
*after* the mtu has been set earlier in ether_ifattach().

PR: 248005