mm/show_mem: optimize si_meminfo_node by reducing redundant code

Refactors the si_meminfo_node() function by reducing redundant code and
improving readability.

Moved the calculation of managed_pages inside the existing loop that
processes pgdat->node_zones, eliminating the need for a separate loop.

Simplified the logic by removing unnecessary preprocessor conditionals.

Ensured that both totalram, totalhigh, and other memory statistics are
consistently set without duplication.

This change results in cleaner and more efficient code without altering
functionality.

Link: https://lkml.kernel.org/r/20250325073803.852594-1-ye.liu@linux.dev


Signed-off-by: Ye Liu <liuye@kylinos.cn>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>