net: Add skb_dstref_steal and skb_dstref_restore

net: Add skb_dstref_steal and skb_dstref_restore

Going forward skb_dst_set will assert that skb dst_entry
is empty during skb_dst_set to prevent potential leaks. There
are few places that still manually manage dst_entry not using
the helpers. Convert them to the following new helpers:
- skb_dstref_steal that resets dst_entry and returns previous dst_entry
  value
- skb_dstref_restore that restores dst_entry previously reset via
  skb_dstref_steal

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250818154032.3173645-2-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>