mm: support pagecache limit

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8NIKC



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Add /pros/sys/vm/cache_limit_mbytes to set page cache limit. This
interface set the upper limit of page cache, if usage of page cache
is over cache_limit_mbytes, it will trigger memory reclaim, the
reclaim size and reclaim interval are decided by interfaces
/proc/sys/vm/cache_reclaim_s and /proc/sys/vm/cache_reclaim_weight,
these two intefaces are introduced in previous patch.

Signed-off-by: Ze Zuo <zuoze1@huawei.com>