etmem: add original kernel swap enabled options

euleros inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4QVXW


CVE: NA

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etmem, the memory vertical expansion technology,
uses DRAM and high-performance storage new media to form multi-level
memory storage.
By grading the stored data, etmem migrates the classified cold
storage data from the storage medium to the high-performance
storage medium,
so as to achieve the purpose of memory capacity expansion and
memory cost reduction.

When the memory expansion function etmem is running, the native
swap function of the kernel needs to be disabled in certain
scenarios to avoid the impact of kernel swap.

This feature provides the preceding functions.

The /sys/kernel/mm/swap/ directory provides the kernel_swap_enable
sys interface to enable or disable the native swap function
of the kernel.

The default value of /sys/kernel/mm/swap/kernel_swap_enable is true,
that is, kernel swap is enabled by default.

Turn on kernel swap:
	echo true > /sys/kernel/mm/swap/kernel_swap_enable

Turn off kernel swap:
	echo false > /sys/kernel/mm/swap/kernel_swap_enable

Signed-off-by: liubo <liubo254@huawei.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>