mm/swapfile: introduce per-memcg swapfile control

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7CGGT


CVE: NA

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With memory.swapfile interface, the avail swap device can be limit for
memcg. The acceptable parameters are 'all', 'none' and valid swap device.
Usage:
	echo /dev/zram0 > memory.swapfile

If the swap device is offline, the swapfile will be fallback to 'none'.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>