userswap: add handling of ZERO_PAGE

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



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We call follow_page() with FOLL_DUMP to handle ZERO_PAGE. Although
FOLL_DUMP is intended for get_dump_page(), it just so happens that its
special treatment of the ZERO_PAGE (returning an error instead of doing
get_page) suits uswap very well. If somehow an abnormal page has sneaked
into the range, we won't oops here.

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