mm/swapfile: fix broken kabi in swap_info_struct

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I645JI



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

This patch fix kabi problem introduced by commit
("mm/swapfile:use percpu_ref to serialize against concurrent swapoff").

Considering swap_info_struct is referenced by pointer and dont use
in third module, so use KABI_EXTEND for two new member variables in
swap_info_struct. Besides, dont remove unused enum value SWP_VALID,
avoid unnecessary undefined alarms.

Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Reviewed-by: Kefeng <Wang&lt;wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>