CVE-2026-74260: netfilter: nf_dup_netdev: add nf_dev_xmit_recursion*() helpers and use them

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nfdupnetdev: add nfdevxmitrecursion() helpers and use them

Update nftdup and nftfwd to use the nfdevxmitrecursion() helpers. This patch also disables BH when transmitting the skb to address a possible migration to different CPU leading to imbalanced decrementation of the recursion counters.

This is modeled after Florian Westphal's devxmitrecursion() API available since commit 97cdcf37b57e ("net: place xmit recursion in softnet data") according to its current state in the tree.