userswap: fix BUG_ON in __mcopy_atomic()

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



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When the swap-in buffer contains no physical pages, the errno in
mfill_atomic_pte_nocopy() will be ENOENT. A BUG_ON error will occur
because the userswap feature does not use the struct page *page and page
is set to NULL.
To fix this issue, the errno should be changed from ENOENT to EINVAL.

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