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.
While here, change terrifying "Invalid radix node head, rn:" message,
to different non-understandable "netcred already exists for given addr/mask",
but less terrifying. Since we know that rn_addaddr() returns NULL if
the same record already exists, we should provide more friendly error.
MFC after: 1 month
c2069a39a4