mm/khugepaged: use start_addr/addr for improved readability

When collapsing a pmd, there are two address in use:

  * address points to the start of pmd
  * address points to each individual page

Current naming makes it difficult to distinguish these two and is hence
error prone.

Considering the plan to collapse mTHP, name the first one `start_addr' and
the second `addr' for better readability and consistency.

Link: https://lkml.kernel.org/r/20250922140938.27343-1-richard.weiyang@gmail.com


Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Nico Pache <npache@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>