CVE-2026-68296: net: gre: fix lltx regression for GRE tunnels with SEQ/CSUM

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

net: gre: fix lltx regression for GRE tunnels with SEQ/CSUM

Before commit 00d066a4d4ed ("netdevfeatures: convert NETIFFLLTX to dev->lltx"), NETIFFLLTX was set unconditionally in both gretunnelinit() and ip6gretnlinitfeatures() alongside GREFEATURES:

dev->features |= GREFEATURES | NETIFFLLTX;

When that commit converted NETIFFLLTX to the dev->lltx flag, it placed 'dev->lltx = true' after the SEQ/CSUM early returns instead of before them. This causes GRE/GRETAP/ip6gre tunnels with SEQ or CSUM+encap to lose lockless TX, reintroducing xmitlock acquisition around their ndostartxmit. Since GRE xmit re-enters the stack via iptunnelxmit(), holding xmitlock risks ABBA deadlock with the underlay device.

CPU0 CPU1 ---- ---- lock(&qdiscxmitlockkey#6); lock(&qdiscxmitlockkey#3); lock(&qdiscxmitlockkey#6); lock(&qdiscxmitlockkey#3);

Fix by moving dev->lltx = true before the early returns in both functions, restoring the original unconditional behavior.