memcg: fix use-after-free of mm_struct

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8LKYV



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Increment the mm_users of mm_struct to avoid use-after-free.

Fixes: 9bbb63c8 ("memcg: introduce per-memcg swapin interface")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>