mm/memmap: prevent double scanning of memmap by kmemleak

kmemleak explicitly scans the mem_map through the valid struct page
objects.  However, memmap_alloc() was also adding this memory to the gray
object list, causing it to be scanned twice.  Remove memmap_alloc() from
the scan list and add a comment to clarify the behavior.

Link: https://lore.kernel.org/lkml/CAOm6qn=FVeTpH54wGDFMHuCOeYtvoTx30ktnv9-w3Nh8RMofEA@mail.gmail.com/
Link: https://lkml.kernel.org/r/20250106021126.1678334-1-guoweikang.kernel@gmail.com


Signed-off-by: Guo Weikang <guoweikang.kernel@gmail.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>