!786 Support userswap feature

Merge Pull Request from: @anred 
 
This patch series optimizes userswap mainly including swap-in and
swap-out.

We tested the concurrent scenario of multi-threaded page fault and
multi-threaded swap-in in the uswap demo;and the remapping in the
swap-out phase and the copy-free function in the swap-in phase were ok.
During the test, related debugging functions including CONFIG_DEBUG_VM,
lockdep, slub debug, kasan and kmemleak are enabled. 
 
Link:https://gitee.com/openeuler/kernel/pulls/786

 

Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Jialin Zhang <zhangjialin11@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>