kernel/nfs: Remove old hack abusing dom_rtattach.

kernel/nfs: Remove old hack abusing dom_rtattach.

* Mostly obtained from FreeBSD. Here's the relevant part of the
  original commit message.

    According to IANA RPC uaddr registry, there are no AFs
    except IPv4 and IPv6, so it's not worth being too abstract here.

    Remove ne_rtable[AF_MAX+1] and use explicit per-AF radix tries.
    Use own initialization without relying on domattach code.

    While I admit that this was one of the rare places in kernel
    networking code which really was capable of doing multi-AF
    without any AF-depended code, it is not possible anymore to
    rely on dom* code.

* Give netexports their own mask radix tree.

Reviewed-by: sephe, dillon
Tested-by: dillon