netfilter: flowtable: fix incorrect Kconfig dependencies

Remove default to 'y', this infrastructure is not fundamental for the
flowtable operational.

Add a missing dependency on CONFIG_NF_FLOW_TABLE.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Fixes: b0381776 ("netfilter: nf_flow_table: count pending offload workqueue tasks")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>