netfilter: flowtable: Free block_cb when being deleted

Free block_cb memory when asked to be deleted.

Fixes: 978703f4

 ("netfilter: flowtable: Add API for registering to flow table events")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>