mm/zswap: delay the initializaton of zswap until the first enablement

hulk inclusion
category: performance
bugzilla: 187468, https://gitee.com/openeuler/kernel/issues/I61HSG



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The zswap initialization consumes some memory. If we don't use zswap,
there's no need to initial it. So delay the initializaton of zswap until
the first enablement.

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