netfilter: Switch to skb_dstref_steal to clear dst_entry

netfilter: Switch to skb_dstref_steal to clear dst_entry

Going forward skb_dst_set will assert that skb dst_entry
is empty during skb_dst_set. skb_dstref_steal is added to reset
existing entry without doing refcnt. Switch to skb_dstref_steal
in ip[6]_route_me_harder and add a comment on why it's safe
to skip skb_dstref_restore.

Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250818154032.3173645-4-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>