mm: sharepool: check size=0 in mg_sp_make_share_k2u()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5QQPG


CVE: NA

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Add a size-0-check in mg_sp_make_share_k2u() to avoid passing 0-size spa
to __insert_sp_area().

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