SUNRPC: Add hex-formatted address support to rpc_peeraddr2str()

Add support for the NFS client's need to export volume information
with IP addresses formatted in hex instead of decimal.

This isn't used yet, but subsequent patches (not in this series) will
change the NFS client to use this functionality.

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