mm: fix shrink nr.unqueued_dirty counter issue

It is needed to ensure sc->nr.unqueued_dirty > 0, which can avoid setting
PGDAT_DIRTY flag when sc->nr.unqueued_dirty and sc->nr.file_taken are both
zero.

Link: https://lkml.kernel.org/r/20240112012353.1387-1-justinjiang@vivo.com


Signed-off-by: Zhiguo Jiang <justinjiang@vivo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>