ksm: convert ksm_mm_slot.link to ksm_mm_slot.hash

In order to use common struct mm_slot, convert ksm_mm_slot.link to
ksm_mm_slot.hash in advance, no functional change.

Link: https://lkml.kernel.org/r/20220831031951.43152-7-zhengqi.arch@bytedance.com


Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>