Factor out domain_over_bg_thresh from wb_over_bg_thresh to remove repeated
code.

Link: https://lkml.kernel.org/r/20240514125254.142203-4-shikemeng@huaweicloud.comSigned-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>