userswap: introduce MREMAP_USWAP_SET_PTE to remap for swapping out

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



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We introduce MREMAP_USWAP_SET_PTE to implement remapping in the swap-out
phase. Unmap the pages between 'addr ~ addr+old_len' and remap them to
'new_addr ~ new_addr+new_len'. During unmapping, the PTE of old_addr is
set to SWP_USERSWAP_ENTRY.

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