userswap: introduce UFFDIO_COPY_MODE_DIRECT_MAP to map without copying

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



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Add a new UFFDIO_COPY mode UFFDIO_COPY_MODE_DIRECT_MAP to map physical
pages without copy_from_user().
We introduce uswap_unmap_anon_page() to unmap an anonymous page and
uswap_map_anon_page() to map page to src addr. We also introduce
mfill_atomic_pte_nocopy() to achieve zero copy by unmapping src_addr to the
physical page and establishing the mapping from dst_addr to the physical
page.

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