userswap: fix VM_BUG_ON() in handle_userfault()

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



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When CONFIG_VM_BUG_ON=y and userswap feature is used, there is a kernel
BUG in handle_userfault(). VM_BUG_ON() didn't allow more than one reason
flag.
Fix this by skipping VM_BUG_ON() if reason is VM_UFFD_MISSING|VM_USWAP.

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