netfilter: flowtable: Make nf_flow_table_offload_add/del_cb inline

Currently, nf_flow_table_offload_add/del_cb are exported by nf_flow_table
module, therefore modules using them will have hard-dependency
on nf_flow_table and will require loading it all the time.

This can lead to an unnecessary overhead on systems that do not
use this API.

To relax the hard-dependency between the modules, we unexport these
functions and make them static inline.

Fixes: 978703f4

 ("netfilter: flowtable: Add API for registering to flow table events")
Signed-off-by: Alaa Hleihel <alaa@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>