mkdir in /dev causes crash

Hi. If I create a directory in /dev, the kernel crashes at

 kern_mkdir+0x293: movl 0x60,%ecx

(Sorry, I forgot to take the console output to work, so I don't have
much more info.)

I guess this is related to changes made to /sys/fs/devfs/devfs_vnops.c
et al?

I'll try to put this into a PR, unless someone can spot immediately where
the problem lies.

Cheers
Michiel