memcg: introduce per-memcg swapin interface

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


CVE: NA

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Add a new per-memcg swapin interface to load data into memory in advance
to improve access efficiency.
Usage:
	# echo 0 > memory.force_swapin

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