SUNRPC: Move the svc_rpcb_cleanup() call sites

Clean up: because svc_rpcb_cleanup() and svc_xprt_destroy_all()
are always invoked in pairs, we can deduplicate code by moving
the svc_rpcb_cleanup() call sites into svc_xprt_destroy_all().

Tested-by: Olga Kornievskaia <okorniev@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>