[PATCH] clean up argument passing in writeback paths

[PATCH] clean up argument passing in writeback paths

The writeback code paths which walk the superblocks and inodes are
getting an increasing arguments passed to them.

The patch wraps those args into the new `struct writeback_control',
and uses that instead.  There is no functional change.

The new writeback_control structure is passed down through the
writeback paths in the place where the old `nr_to_write' pointer used
to be.

writeback_control will be used to pass new information up and down the
writeback paths.  Such as whether the writeback should be non-blocking,
and whether queue congestion was encountered.