mm/vmscan: simplify shrink_node()

The difference between sc->nr_reclaimed and nr_reclaimed is computed three
times.  Introduce a new variable to record the value, so it only needs to
be computed once.

Link: https://lkml.kernel.org/r/20230411061757.12041-1-haifeng.xu@shopee.com


Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>