writeback: move over_bground_thresh() to mm/page-writeback.c

and rename it to wb_over_bg_thresh().  The function is closely tied to
the dirty throttling mechanism implemented in page-writeback.c.  This
relocation will allow future updates necessary for cgroup writeback
support.

While at it, add function comment.

This is pure reorganization and doesn't introduce any behavioral
changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>