lk: add mkfifo support

[Alex Chiang]

* Alex Chiang <achiang at hp.com>:
> 
> Tested with the Linux POSIX file system test suite:
> 
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/00......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/01......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/02......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/03......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/04......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/05......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/06......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/07......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/08......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/09......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/10......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/11......ok                       
> /root/crfs/pjd-fstest-20080402/tests/mkfifo/12......ok                       

Hrm, it turns out that I didn't exactly have a crfs volume
mounted when I did this test. :-/

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       
        Failed 2/36 tests, 94.44% okay
/root/crfs/pjd-fstest-20080402/tests/mkfifo/01......ok                       
/root/crfs/pjd-fstest-20080402/tests/mkfifo/02......FAILED tests 1-2         
        Failed 2/3 tests, 33.33% okay
/root/crfs/pjd-fstest-20080402/tests/mkfifo/03......FAILED tests 1-4, 8-11   
        Failed 8/11 tests, 27.27% okay
/root/crfs/pjd-fstest-20080402/tests/mkfifo/04......ok                       
/root/crfs/pjd-fstest-20080402/tests/mkfifo/05......FAILED test 12           
        Failed 1/12 tests, 91.67% okay
/root/crfs/pjd-fstest-20080402/tests/mkfifo/06......FAILED test 12           
        Failed 1/12 tests, 91.67% okay
/root/crfs/pjd-fstest-20080402/tests/mkfifo/07......ok                       
/root/crfs/pjd-fstest-20080402/tests/mkfifo/08......ok                       
/root/crfs/pjd-fstest-20080402/tests/mkfifo/09......ok                       
/root/crfs/pjd-fstest-20080402/tests/mkfifo/10......ok                       
/root/crfs/pjd-fstest-20080402/tests/mkfifo/11......ok                       
/root/crfs/pjd-fstest-20080402/tests/mkfifo/12......ok                       

/ac