io_uring:drop identity before creating a private one

hulk inclusion
category: bugfix
bugzilla: 187387, https://gitee.com/openeuler/kernel/issues/I5KTEM


CVE: NA

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When we need to create a private copy of io_identity, we will
get the use of current task's private structs, whitch means we
should increase their reference counts. If we have grabed some
other structs before, we should drop them and clear related
flags. Otherwise, leak of old structs and uaf of new structs
may occur.

Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>