CVE-2026-31691: igb: remove napi_synchronize() in igb_down()

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

igb: remove napisynchronize() in igbdown()

When an AFXDP zero-copy application terminates abruptly (e.g., kill -9), the XSK buffer pool is destroyed but NAPI polling continues. igbcleanrxirqzc() repeatedly returns the full budget, preventing napicompletedone() from clearing NAPISTATESCHED.

igbdown() calls napisynchronize() before napidisable() for each queue vector. napisynchronize() spins waiting for NAPISTATESCHED to clear, which never happens. igbdown() blocks indefinitely, the TX watchdog fires, and the TX queue remains permanently stalled.

napidisable() already handles this correctly: it sets NAPISTATEDISABLE. After a full-budget poll, napipoll() checks napidisablepending(). If set, it forces completion and clears NAPISTATESCHED, breaking the loop that napisynchronize() cannot.

napisynchronize() was added in commit 41f149a285da ("igb: Fix possible panic caused by Rx traffic arrival while interface is down"). napidisable() provides stronger guarantees: it prevents further scheduling and waits for any active poll to exit. Other Intel drivers (ixgbe, ice, i40e) use napidisable() without a preceding napisynchronize() in their down paths.

Remove redundant napisynchronize() call and reorder napidisable() before igbsetqueuenapi() so the queue-to-NAPI mapping is only cleared after polling has fully stopped.