Do not allow creation of char or block special nodes with VNOVAL dev_t.

As was reported on http://seclists.org/oss-sec/2016/q3/68, tmpfs code
contains assertion that rdev != VNOVAL. On FreeBSD, there is no other
consequences except triggering the assert. To be compatible with
systems where device nodes have some significance, reject mknod(2)
call with dev == VNOVAL at the syscall level.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week