SUNRPC: Rename xs_format_peer_addresses

Prepare to add an IPv6 version of xs_format_peer_addresses by renaming it
to xs_format_ipv4_peer_addresses.

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>