SUNRPC: Free address buffers in a loop

Use more generic logic to free buffers holding formatted addresses.  This
makes it less likely a bug will be introduced when adding additional buffer
types in xs_format_peer_address().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>