etmem: fix the div 0 problem in swapcache reclaim process

euleros inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7JI6K


CVE: NA

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In the swapcache recycling process, the number of pages
to be reclaimed on each node is obtained as follows:

nr_to_reclaim[nid_num] = (swapcache_to_reclaim / (swapcache_total_reclaimable / nr[nid_num]));

However, nr[nid_num] is obtained by traversing the number
of swapcache pages on each node.
If there are multiple nodes in the environment and
no swap process occurs on a node, no swapcache page exists.
The value of nr[nid_num] may be 0.

Therefore, division by zero errors may occur.

Signed-off-by: liubo <liubo254@huawei.com>