mm: bdi: initialize bdi_min_ratio when bdi is unregistered

Initialize min_ratio if it is set during bdi unregistration.  This can
prevent problems that may occur a when bdi is removed without resetting
min_ratio.

For example.
1) insert external sdcard
2) set external sdcard's min_ratio 70
3) remove external sdcard without setting min_ratio 0
4) insert external sdcard
5) set external sdcard's min_ratio 70 << error occur(can't set)

Because when an sdcard is removed, the present bdi_min_ratio value will
remain.  Currently, the only way to reset bdi_min_ratio is to reboot.

[akpm@linux-foundation.org: tweak comment and coding style]

Link: https://lkml.kernel.org/r/20211021161942.5983-1-mj0123.lee@samsung.com
Signed-off-by: Manjong Lee <mj0123.lee@samsung.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Changheun Lee <nanich.lee@samsung.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <seunghwan.hyun@samsung.com>
Cc: <sookwan7.kim@samsung.com>
Cc: <yt0928.kim@samsung.com>
Cc: <junho89.kim@samsung.com>
Cc: <jisoo2146.oh@samsung.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Manjong Lee

mm/backing-dev.c

@@ -945,6 +945,13 @@ void bdi_unregister(struct backing_dev_info *bdi)
 	wb_shutdown(&bdi->wb);
 	cgwb_bdi_unregister(bdi);
 
+	/*
+	 * If this BDI's min ratio has been set, use bdi_set_min_ratio() to
+	 * update the global bdi_min_ratio.
+	 */
+	if (bdi->min_ratio)
+		bdi_set_min_ratio(bdi, 0);
+
 	if (bdi->dev) {
 		bdi_debug_unregister(bdi);
 		device_unregister(bdi->dev);