net: flow_offload: remove netns parameter from flow_block_cb_alloc()

No need to annotate the netns on the flow block callback object,
flow_block_cb_is_busy() already checks for used blocks.

Fixes: d63db30c

 ("net: flow_offload: add flow_block_cb_alloc() and flow_block_cb_free()")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>