CVE-2026-72248: netfilter: flowtable: support IPIP tunnel with direct xmit

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

netfilter: flowtable: support IPIP tunnel with direct xmit

The combination of IPIP tunnel with direct xmit, eg. bridge device, breaks because no dstentry is provided to check the skb headroom and to set the iph->fragoff field. This leads to invalid dst usage and can trigger a crash in the tunnel transmit path.

Fix this by moving dstcache and dstcookie out of the runtime union so that they can be shared by neighbour, xfrm, and direct tunnel flows. For FLOWOFFLOADXMITDIRECT tuples carrying tunnel metadata, preserve route state in these shared fields and release it through the common dst release path.

Since dstentry is now available to the three supported xmit modes and dstrelease() already deals with NULL dst, remove the xmit type check in nftflowdstrelease(). Moreover, skip the check if the dst entry is NULL in nfflowdstcheck() which is now the case for the direct xmit case.

Based on patch from Rein Wei <n05ec@lzu.edu.cn>.