SUNRPC: Make address format buffers more generic

For now we will assume that all transports will use the address format
buffers in the rpc_xprt struct to store their addresses.  Change
rpc_peer2str() to be a generic routine to handle this, and get rid of the
print_address() op in the rpc_xprt_ops vector.

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