We still have to account for a copyin. Make sure the copyin will
succeed by passing the FreeBSD syscall a pointer to userspace,
albeit one that's automagically mapped into kernel space.
Reported by: mr, Mitsuru IWASAKI <iwasaki@jp.FreeBSD.org>
Tested by: Mitsuru IWASAKI <iwasaki@jp.FreeBSD.org>