mm/khugepaged: remove definition of struct khugepaged_mm_slot

Current code is not correct to get struct khugepaged_mm_slot by
mm_slot_entry() without checking mm_slot is !NULL.  There is no problem
reported since slot is the first element of struct khugepaged_mm_slot.

While struct khugepaged_mm_slot is just a wrapper of struct mm_slot, there
is no need to define it.

Remove the definition of struct khugepaged_mm_slot, so there is not chance
to miss use mm_slot_entry().

[richard.weiyang@gmail.com: fix use-after-free crash]
  Link: https://lkml.kernel.org/r/20250922002834.vz6ntj36e75ehkyp@master
Link: https://lkml.kernel.org/r/20250919071244.17020-3-richard.weiyang@gmail.com


Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Kiryl Shutsemau <kirill@shutemov.name>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>