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

sys/kern/vfs_syscalls.c

@@ -1167,6 +1167,8 @@ kern_mknodat(struct thread *td, int fd, char *path, enum uio_seg pathseg,
 	case S_IFCHR:
 	case S_IFBLK:
 		error = priv_check(td, PRIV_VFS_MKNOD_DEV);
+		if (error == 0 && dev == VNOVAL)
+			error = EINVAL;
 		break;
 	case S_IFMT:
 		error = priv_check(td, PRIV_VFS_MKNOD_BAD);