netfilter: flowtable: skip offload setup if disabled

nftables test case
tests/shell/testcases/flowtable/0001flowtable_0

results in a crash. After the refactor, if we leave early via
nf_flowtable_hw_offload(), then "struct flow_block_offload" is left
in an uninitialized state, but later users assume its initialised.

Fixes: a7965d58 ("netfilter: flowtable: add nf_flow_table_offload_cmd()")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>