lk: add mkfifo support

[Alex Chiang]

* Zach Brown <zach.brown at oracle.com>:
> 
> > But I think the patch is a good enough first step in the right
> > direction, as before, we would hit a BUG() before we even got to
> > making a fifo, and with this patch, we get significantly further.
> > 
> > /root/crfs/pjd-fstest-20080402/tests/mkfifo/00......FAILED tests 33-34
> 
> It'd be cool if we could land one patch which adds fifos that pass all
> the tests..

Fair 'nuff.

> If we find out that these failures are due to some other weird part of
> the system, sure, we can land these fifo adding patches as is.

I'll keep poking.

Thanks.

/ac