9pfs: adjust the order of resource cleanup in device unrealize

Unrealize should undo things that were set during realize in
reverse order. So should do in the error path in realize.

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Greg Kurz <groug@kaod.org>
