userswap: fix some type and logical bugs

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



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As follows, fix some type and logical bugs.
1) The type of index variable is changed from int to unsigned long to
support large memory registration.
2) Fix the bug that USWAP_PAGES_DIRTY does not take effect.
3) Take the mmap_read_lock() when using the VMA in
uswap_adjust_uffd_range().
4) Do some code refactoring and cleancode.

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