netfilter: flowtable: simplify route logic

[ Upstream commit fa502c86

 ]

Grab reference to dst from skbuff earlier to simplify route caching.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Stable-dep-of: 9e0f0430

 ("netfilter: nft_flow_offload: reset dst in route object after setting up flow")
Signed-off-by: Sasha Levin <sashal@kernel.org>