io_uring: COW io_identity on mismatch

If the io_identity doesn't completely match the task, then create a
copy of it and use that. The existing copy remains valid until the last
user of it has gone away.

This also changes the personality lookup to be indexed by io_identity,
instead of creds directly.

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

include/linux/io_uring.h

@@ -15,6 +15,7 @@ struct io_identity {
 	struct nsproxy			*nsproxy;
 	struct fs_struct		*fs;
 	unsigned long			fsize;
+	refcount_t			count;
 };
 
 struct io_uring_task {