nfsd: unregister with rpcbind when deleting a transport

When a listener is added, a part of creation of transport also registers
program/port with rpcbind. However, when the listener is removed,
while transport goes away, rpcbind still has the entry for that
port/type.

When deleting the transport, unregister with rpcbind when appropriate.

---v2 created a new xpt_flag XPT_RPCB_UNREG to mark TCP and UDP
transport and at xprt destroy send rpcbind unregister if flag set.

Suggested-by: Chuck Lever <chuck.lever@oracle.com>
Fixes: d093c908 ("nfsd: fix management of listener transports")
Cc: stable@vger.kernel.org
Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>