=== fix applied to guest /usr/src (never to the host sys/ tree) ===
$ cd /usr/src && patch -p0 < /root/poc/fix.dma-buf.diff
Patching file sys/dev/drm/linux_dma-buf.c using Plan A:
Hunk #1 succeeded at 111.
Hunk #2 succeeded at 162.
$ cd /usr/src/sys/dev/drm/drm && make
cc ... -c /usr/src/sys/dev/drm/drm/../linux_dma-buf.c   (recompiled)
cc -Wl,... -r -Wl,-d -o drm.ko ... linux_dma-buf.o ...
install -o root -g wheel -m 555   drm.ko /boot/kernel
$ reboot