writeback: report dirty thresholds in /proc/vmstat

writeback: report dirty thresholds in /proc/vmstat

The kernel already exposes the user desired thresholds in /proc/sys/vm
with dirty_background_ratio and background_ratio.  But the kernel may
alter the number requested without giving the user any indication that is
the case.

Knowing the actual ratios the kernel is honoring can help app developers
understand how their buffered IO will be sent to the disk.

        $ grep threshold /proc/vmstat
        nr_dirty_threshold 409111
        nr_dirty_background_threshold 818223

Signed-off-by: Michael Rubin <[email protected]>
Cc: Wu Fengguang <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Nick Piggin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>