userswap: add page_count() check for swap-out VA

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



--------------------------------

Add page_count() check for swap-out VA to make sure that no other kernel
mechanism is using this physical page.
Add lru_add_drain_all() before swap-out VA to correct page_count() of
swap-out pages.

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