mm: numa,memblock: Use SZ_1M macro to denote bytes to MB conversion

Replace the manual bitwise conversion of bytes to MB with
SZ_1M macro, a standard macro used within the mm subsystem,
to improve readability.

Signed-off-by: Pratyush Brahma <pratyush.brahma@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250820-numa-memblks-refac-v2-1-43bf1af02acd@oss.qualcomm.com


Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>