netfilter: flowtable: initialise extack before use

Fix missing initialisation of extack in flow offload.

Fixes: c29f74e ("netfilter: nf_flow_table: hardware offload support")
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_flow_table_offload.c

@@ -841,8 +841,8 @@ static int nf_flow_offload_tuple(struct nf_flowtable *flowtable,
 				 struct list_head *block_cb_list)
 {
 	struct flow_cls_offload cls_flow = {};
+	struct netlink_ext_ack extack = {};
 	struct flow_block_cb *block_cb;
-	struct netlink_ext_ack extack;
 	__be16 proto = ETH_P_ALL;
 	int err, i = 0;