mm/page_alloc: find_large_buddy() from start_pfn aligned order

We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy.
While if the order is less than start_pfn aligned order, we would get the
same pfn and do the same check again.

Iterate from start_pfn aligned order to reduce duplicated work.

[richard.weiyang@gmail.com: add comment on assignment of order]
Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
Link: https://lkml.kernel.org/r/20250902025807.11467-1-richard.weiyang@gmail.com
Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
Link: https://lkml.kernel.org/r/20250902025807.11467-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>