xfs: bulkstat chunk-formatter has issues

The loop construct has issues:
	- clustidx is completely unused, so remove it.
	- the loop tries to be smart by terminating when the
	  "freecount" tells it that all inodes are free. Just drop
	  it as in most cases we have to scan all inodes in the
	  chunk anyway.
	- move the "user buffer left" condition check to the only
	  point where we consume space int eh user buffer.
	- move the initialisation of agino out of the loop, leaving
	  just a simple loop control logic using the clusteridx.

Also, double handling of the user buffer variables leads to problems
tracking the current state - use the cursor variables directly
rather than keeping local copies and then having to update the
cursor before returning.

cc: <stable@vger.kernel.org> # 3.17
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>