io_uring: preserve task restrictions across exec

In the Linux kernel, the following vulnerability has been resolved: io_uring: preserve task restrictions across exec Per-task restrictions apply to all rings created by a task. Once installed, they should not be dropped across exec. For a task that has used io_uring, the exec cancellation path calls __io_uring_free(). This frees both the task context and the per-task restriction, so a ring created after exec is unrestricted. Split task context cleanup into io_uring_free_tctx(), and use it from the exec cancellation path. Keep __io_uring_free() for final task cleanup, where both the context and restriction are released.