Sometimes kjournald has to refile a huge number of buffers, because someone
else wrote them out beforehand - they are all clean.

This happens under a lock and scheduling latencies of 88 milliseconds on a
2.7GHx CPU were observed.

The patch forward-ports a little bit of the 2.4 low-latency patch to fix this
problem.

Worst-case on ext3 is now sub-half-millisecond, except for when the RCU
dentry reaping softirq cuts in :(