RDMA/core: always drop device refcount in ib_del_sub_device_and_put()

In the Linux kernel, the following vulnerability has been resolved:

RDMA/core: always drop device refcount in ib_del_sub_device_and_put()

Since nldev_deldev() (introduced by commit 060c642b2ab8 ("RDMA/nldev: Add support to add/delete a sub IB device through netlink") grabs a reference using ib_device_get_by_index() before calling ib_del_sub_device_and_put(), we need to drop that reference before returning -EOPNOTSUPP error.