netfilter: nf_tables: don't fail when updating base chain policy

The following nftables test case fails on nf-next:

tests/shell/run-tests.sh tests/shell/testcases/transactions/0011chain_0

The test case contains:
add chain x y { type filter hook input priority 0; }
add chain x y { policy drop; }"

The new test
if (chain->flags ^ flags)
	return -EOPNOTSUPP;

triggers here, because chain->flags has NFT_BASE_CHAIN set, but flags
is 0 because no flag attribute was present in the policy update.

Just fetch the current flag settings of a pre-existing chain in case
userspace did not provide any.

Fixes: c9626a2c ("netfilter: nf_tables: add hardware offload support")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>