mm: sharepool: delete redundant check in __sp_remap_get_pfn

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5QETC


CVE: NA

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sp_make_share_k2u only supports vmalloc address now. Therefore, delete a
backup handle case.

Signed-off-by: Guo Mengqi <guomengqi3@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>