mm: memory-failure: convert to pr_fmt()

Use pr_fmt to prefix all pr_<level> output, but unpoison_memory() and
soft_offline_page() are used by error injection, which have own prefixes
like "Unpoison:" and "soft offline:", meanwhile, soft_offline_page() could
be used by memory hotremove, so reset pr_fmt before unpoison_pr_info
definition to keep the original output for them.

[wangkefeng.wang@huawei.com: v3]
  Link: https://lkml.kernel.org/r/20220729031919.72331-1-wangkefeng.wang@huawei.com
Link: https://lkml.kernel.org/r/20220726081046.10742-1-wangkefeng.wang@huawei.com
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>