[PATCH 0/4] writeback: tracing and wbc->nr_to_write fixes

This series contains the initial writeback tracing patches from
Jens, as well as the extensions I added to provide visibility into
writeback control structures as the are used by the writeback code.
The visibility given is sufficient to understand what is happening
in the writeback path - what path is writing data, what path is
blocking on congestion, etc, and to determine the differences in
behaviour for different sync modes and calling contexts. This
tracing really needs to be integrated into mainline so that anyone
can improve the tracing as they use it to track down problems
in our convoluted writeback paths.

The remaining patches are fixes to problems that the new tracing
highlighted.