CVE-2026-72493: net: serialize netif_running() check in enqueue_to_backlog()

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

net: serialize netifrunning() check in enqueuetobacklog()

Syzbot reported a KASAN slab-use-after-free in fibruleslookup().

The root cause is a race condition where packets can escape the backlog flushing during device unregistration (e.g., during netns exit).

Commit e9e4dd3267d0 ("net: do not process device backlog during unregistration") introduced a lockless netifrunning() check in enqueuetobacklog() to prevent queuing packets to an unregistering device.

However, this creates a TOCTOU race window.

A lockless transmitter (like vethxmit) can pass the check before devclose() clears IFFUP. If the transmitter is then delayed, flushallbacklogs() can run and finish before the transmitter grabs the backlog lock and queues the packet. The packet then escapes the flush and triggers UAF later when processed.

Fix this by moving the netifrunning() check inside the backlog lock. This serializes the check with the flush work (which also grabs the lock). We then either queue the packet before the flush runs (so it gets flushed), or check netifrunning() after the flush/close completes (so it gets dropped).