SUNRPC: Introduce a specific rpcb_create for contacting localhost

Add rpcb_create_local() for use by rpcb_register() and upcoming IPv6
registration functions.

Ensure any errors encountered by rpcb_create_local() are properly
reported.

We can also use a statically allocated constant loopback socket address
instead of one allocated on the stack and initialized every time the
function is called.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>