netfilter: flowtable: initialise extack before use

Fix missing initialisation of extack in flow offload.

Fixes: c29f74e0 ("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>