BACKPORT: FROMLIST: mm/vmscan.c: refactor shrink_node()

This patch refactors shrink_node(). This will make the upcoming
changes to mm/vmscan.c more readable.

Signed-off-by: Yu Zhao <yuzhao@google.com>
Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
(am from https://lore.kernel.org/patchwork/patch/1478356/)

BUG=b:123039911
TEST=Built

Change-Id: Ic940290e51d97101af07db01365ac59f46071719
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3132020


Tested-by: Yu Zhao <yuzhao@chromium.org>
Commit-Queue: Yu Zhao <yuzhao@chromium.org>
Reviewed-by: Alexandre Marciano Gimenez <raging@google.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>