SUNRPC: Register both netids for AF_INET6 servers

TI-RPC is a user-space library of RPC functions that replaces ONC RPC
and allows RPC to operate in the new world of IPv6.

TI-RPC combines the concept of a transport protocol (UDP and TCP)
and a protocol family (PF_INET and PF_INET6) into a single identifier
called a "netid."  For example, "udp" means UDP over IPv4, and "udp6"
means UDP over IPv6.

For rpcbind, then, the RPC service tuple that is registered and
advertised is:

  [RPC program, RPC version, service address and port, netid]

instead of

  [RPC program, RPC version, port, protocol]

Service address is typically ANYADDR, but can be a specific address
of one of the interfaces on a multi-homed host.  The third item in
the new tuple is expressed as a universal address.

The current Linux rpcbind implementation registers a netid for both
protocol families when RPCB_SET is done for just the PF_INET6 version
of the netid (ie udp6 or tcp6).  So registering "udp6" causes a
registration for "udp" to appear automatically as well.

We've recently determined that this is incorrect behavior.  In the
TI-RPC world, "udp6" is not meant to imply that the registered RPC
service handles requests from AF_INET as well, even if the listener
socket does address mapping.  "udp" and "udp6" are entirely separate
capabilities, and must be registered separately.

The Linux kernel, unlike TI-RPC, leverages address mapping to allow a
single listener socket to handle requests for both AF_INET and AF_INET6.
This is still OK, but the kernel currently assumes registering "udp6"
will cover "udp" as well.  It registers only "udp6" for it's AF_INET6
services, even though they handle both AF_INET and AF_INET6 on the same
port.

So svc_register() actually needs to register both "udp" and "udp6"
explicitly (and likewise for TCP).  Until rpcbind is fixed, the
kernel can ignore the return code for the second RPCB_SET call.

Please merge this with commit 15231312:

    SUNRPC: Support IPv6 when registering kernel RPC services

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Olaf Kirch <okir@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>