vmscan, cleanup: add for_each_managed_zone_pgdat macro

vmscan, cleanup: add for_each_managed_zone_pgdat macro

The macro is introduced to eliminate redundancy in the repeated iteration
over managed zones in pgdat data structure, reducing the potential for
errors. This change doesn't introduce any functional modifications.
Due to concentration of the pattern in vmscan.c the macro is placed
locally in that file.

Link: https://lkml.kernel.org/r/20250210160818.686-1-bertrand.wlodarczyk@intel.com
Signed-off-by: Bertrand Wlodarczyk <bertrand.wlodarczyk@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>