SUNRPC: add a function to format IPv6 addresses

Clone xs_format_ipv4_peer_addresses into an IPv6 version.

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>