DF-4772 fix-validation kernel build (guest /usr/src after applying fix.diff)
=============================================================================
command : cd /usr/src && make -j6 nativekernel   (log: /root/build4772.log)
result  : RC=0 under -Werror, 4299 cc invocations
          last lines:
          --------------------------------------------------------------
          RC=0
install : cd /usr/src && make installkernel -> RC=0
          ">>> Kernel install for X86_64_GENERIC completed on Wed Sep 23 13:19:55 UTC 2026"
reboot  : into "DragonFly 6.5-DEVELOPMENT #1: Wed Sep 23 13:15:01 UTC 2026"
patch   : grep -n "else if (fence_excl)" /usr/src/sys/dev/drm/linux_reservation.c -> 412: else if (fence_excl)
re-run  : identical PoC trigger -> run.fixed.log ("BALANCED")
cleanup : vm.sh reset with-src (full /root/build4772.log lived on the build
          disk image and was reverted with it; the decisive tail above is
          verbatim from the build session)