io_uring: store io_identity in io_uring_task

This is, by definition, a per-task structure. So store it in the
task context, instead of doing carrying it in each io_kiocb. We're being
a bit inefficient if members have changed, as that requires an alloc and
copy of a new io_identity struct. The next patch will fix that up.

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

include/linux/io_uring.h

@@ -24,6 +24,7 @@ struct io_uring_task {
 	struct wait_queue_head	wait;
 	struct file		*last;
 	atomic_long_t		req_issue;
+	struct io_identity	identity;
 
 	/* completion side */
 	bool			in_idle ____cacheline_aligned_in_smp;