DF-4529 fix kernel builds (guest, gcc 8.3, -Werror):

== build-2 (fix v1) ==
>>> Kernel build for X86_64_GENERIC completed on Mon Sep 21 05:32:51 UTC 2026
>>> Kernel install for X86_64_GENERIC completed on Mon Sep 21 05:38:26 UTC 2026
nm: ffffffff80311bc0 t nvme_pci_detach   (was ffffffff80311920 = unpatched)

== build-3 (fix v2) ==
>>> Kernel build for X86_64_GENERIC started on Mon Sep 21 05:56:00 UTC 2026
>>> Kernel build for X86_64_GENERIC completed on Mon Sep 21 06:05:04 UTC 2026
>>> Kernel install for X86_64_GENERIC completed on Mon Sep 21 06:07:53 UTC 2026
(a first installkernel aborted on 0-byte .ko debris from a killed install;
 debris removed with find /boot/kernel -maxdepth 1 -size 0 -delete, install clean)