mm/page-writeback: consolidate wb_thresh bumping logic into __wb_calc_thresh

mm/page-writeback: consolidate wb_thresh bumping logic into __wb_calc_thresh

Address the feedback from 39ac99852fca ("mm/page-writeback: raise
wb_thresh to prevent write blocking with strictlimit)".  The wb_thresh
bumping logic is scattered across wb_position_ratio, __wb_calc_thresh, and
wb_update_dirty_ratelimit.  For consistency, consolidate all wb_thresh
bumping logic into __wb_calc_thresh.

Link: https://lkml.kernel.org/r/20241121100539.605818-1-jimzhao.ai@gmail.com
Signed-off-by: Jim Zhao <jimzhao.ai@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>