net: flow_offload: add flow_block_cb_is_busy() and use it

This patch adds a function to check if flow block callback is already in
use.  Call this new function from flow_block_cb_setup_simple() and from
drivers.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>