userswap: fix variable uninitialized in uswap_unmap_anon_page()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CAIM



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When the pte is none and old_pte is not NULL, _old_pte is uninitialized
and will be passed out of uswap_unmap_anon_page().
To fix this, add a return value to uswap_unmap_anon_page to indicate
whether the pte is none before unmapping.

Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>