mm/vmscan: modify the assignment logic of the scan and total_scan variables

The scan and total_scan variables can be initialized to 0 when they are
defined, replacing the separate assignment statements.

Link: https://lkml.kernel.org/r/20250417092422.1333620-1-hao.ge@linux.dev


Signed-off-by: Hao Ge <gehao@kylinos.cn>
Acked-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>