mm/memory.c: update the first page in clear_gigantic_page_chunk

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I57SM0


CVE: NA

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Patch "mm: parallelize clear_gigantic_page" make clear_gigantic_page
to be parallelized. But forgot to update the first page which results the
first page for each block is still the head page. Fix it by calculating
the first page for each block.

By the way, add a check to pointer p in order to prevent kernel panic.

Fixes: ae0cd4d4 ("mm: parallelize clear_gigantic_page")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>