lk: add mkfifo support

[Zach Brown]

> 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..

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.

- z