Build of the single-fix ng_iface.ko inside the guest (module rebuild only;
kernel untouched). /usr/src = pristine tree + fix.diff.

    # cd /usr/src && patch -p1 < /tmp/fixA.diff
    Patching file sys/netgraph/iface/ng_iface.c using Plan A.
    Hunk #1 succeeded at 287.
    # cd sys/netgraph/iface && make
    cc -O2 -pipe -D_KERNEL -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes
       -Wmissing-prototypes -Wpointer-arith -Wcast-qual -Wold-style-definition
       -Wmissing-include-dirs -Wno-pointer-sign -Winit-self -Wundef
       -Wno-unused-but-set-variable -std=gnu11 -Werror -DKLD_MODULE -nostdinc
       -I. -Idragonfly -I/usr/obj/usr/src/sys/netgraph/iface/forwarder_x86_64/include
       -Idragonfly/../include -Wold-style-declaration -fno-common -ffreestanding
       -fno-stack-protector -fno-strict-aliasing -fno-strict-overflow
       -fno-asynchronous-unwind-tables -fno-omit-frame-pointer -mcmodel=kernel
       -mno-red-zone -mno-mmx -mno-sse -msoft-float -mno-fp-ret-in-387
       -mindirect-branch=thunk-inline -c /usr/src/sys/netgraph/iface/ng_iface.c
    cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv -r -Wl,-d -o ng_iface.ko ng_iface.o

(compile clean, -Werror, no warnings; identical flags to the shipped module)