SUNRPC: Add IPv6 address support to net/sunrpc/xprtsock.c

Finalize support for setting up RPC client transports to remote RPC
services addressed via IPv6.

Based on work done by Gilles Quillard at Bull Open Source.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Aurelien Charbon <aurelien.charbon@ext.bull.net>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>