mm/vmscan: add drop_caches_loop_limit to break loop in drop_slab_node

hulk inclusion
category: bugfix
bugzilla: 175105
CVE: NA

-------------------------------------------------

Command 'echo 2 > /proc/sys/vm/drop_caches' could lead to a loop in
drop_slab_node for a long while. Add /proc/sys/vm/drop_caches_loop_limit
to break loop in drop_slab_node.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: tong tiangen <tongtiangen@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>