SUNRPC: Add API to acquire source address

NFSv4.0 clients must send endpoint information for their callback
service to NFSv4.0 servers during their first contact with a server.
Traditionally on Linux, user space provides the callback endpoint IP
address via the "clientaddr=" mount option.

During an NFSv4 migration event, it is possible that an FSID may be
migrated to a destination server that is accessible via a different
source IP address than the source server was.  The client must update
callback endpoint information on the destination server so that it can
maintain leases and allow delegation.

Without a new "clientaddr=" option from user space, however, the
kernel itself must construct an appropriate IP address for the
callback update.  Provide an API in the RPC client for upper layer
RPC consumers to acquire a source address for a remote.

The mechanism used by the mount.nfs command is copied: set up a
connected UDP socket to the designated remote, then scrape the source
address off the socket.  We are careful to select the correct network
namespace when setting up the temporary UDP socket.

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