smb: client: make sure smbd_disconnect_rdma_work() doesn't run after smbd_destroy() took over

If we're already disconnecting we don't need to queue the
disconnect_work again. disable_work() turns the next queue_work()
into a no-op.

Also let smbd_destroy() cancel(and disable) queued disconnect_work and
call smbd_disconnect_rdma_work() inline.

The makes it more obvious that disconnect_work is never queued
again after smbd_destroy() called smbd_disconnect_rdma_work().

It also means we have a single place to call rdma_disconnect().

While there we better also disable all other [delayed_]work.

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>