userswap: add a new flag 'MAP_REPLACE' for mmap()

hulk inclusion
category: feature
bugzilla: 47439
CVE: NA

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

To make sure there are no other userspace threads access the memory
region we are swapping out, we need unmmap the memory region, map it
to a new address and use the new address to perform the swapout. We add
a new flag 'MAP_REPLACE' for mmap() to unmap the pages of the input
parameter 'VA' and remap them to a new tmpVA.

Signed-off-by: Guo Fan <guofan5@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>