mount.nfs: Create a new API to find out client's address
Provide a version of clnt_ping() that discovers the client's address, but
doesn't do an RPC ping. The in-kernel text-based mount code already does
a ping, so all we need here is address discovery.
As well, add a block comment in front of clnt_ping() that hopefully
elucidates the differences.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com> Signed-off-by: Neil Brown <neilb@suse.de>