net: cls_api: Fix uninitialised struct field bo->unlocked_driver_cb

The 'unlocked_driver_cb' struct field in 'bo' is not being initialized
in tcf_block_offload_init(). The uninitialized 'unlocked_driver_cb'
will be used when calling unlocked_driver_cb(). So initialize 'bo' to
zero to avoid the issue.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 0fdcf78d

 ("net: use flow_indr_dev_setup_offload()")
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>