mmun_start indicates mmu_notifier start address but there's no mmu_notifier
stuff in remove_migration_pmd. This will make it hard to get the meaning of
mmun_start. Rename it to haddr to avoid confusing readers and also imporve
readability.

Link: https://lkml.kernel.org/r/20220704132201.14611-7-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>