sw64: mm: fix pmd_present() for THP

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I73O8A



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When splitting a pmd, _PAGE_VALID will be temporarily cleared, which
results in pmd_present() return false.

Since _PAGE_PSE will remain set, check it to avoid this issue.

Signed-off-by: Mao Minkai <maominkai@wxiat.com>
Reviewed-by: He Sheng <hesheng@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>