io-wq: inherit audit loginuid and sessionid

Make sure the async io-wq workers inherit the loginuid and sessionid from
the original task, and restore them to unset once we're done with the
async work item.

While at it, disable the ability for kernel threads to write to their own
loginuid.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring.h

@@ -15,6 +15,10 @@ struct io_identity {
 	struct nsproxy			*nsproxy;
 	struct fs_struct		*fs;
 	unsigned long			fsize;
+#ifdef CONFIG_AUDIT
+	kuid_t				loginuid;
+	unsigned int			sessionid;
+#endif
 	refcount_t			count;
 };