SUNRPC: Fix up svc_unregister()

With the new rpcbind code, a PMAP_UNSET will not have any effect on
services registered via rpcbind v3 or v4.

Implement a version of svc_unregister() that uses an RPCB_UNSET with
an empty netid string to make sure we have cleared *all* entries for
a kernel RPC service when shutting down, or before starting a fresh
instance of the service.

Use the new version only when CONFIG_SUNRPC_REGISTER_V4 is enabled;
otherwise, the legacy PMAP version is used to ensure complete
backwards-compatibility with the Linux portmapper daemon.

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