userswap: move userswap feature code into mm/userswap.c

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CAIM



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This patch moves the code related to enable_userswap and CONFIG_USERSWAP
to mm/userswap.c. This allows for better encapsulation and easier
maintenance.

Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>