io_uring: comment on kfree(iovec) checks

kfree() handles NULL pointers well, but io_{read,write}() checks it
because of performance reasons. Leave a comment there for those who are
tempted to patch it.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3204,6 +3204,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
 	kiocb_done(kiocb, ret, cs);
 	ret = 0;
 out_free:
+	/* it's reportedly faster than delegating the null check to kfree() */
 	if (iovec)
 		kfree(iovec);
 	return ret;
@@ -3300,6 +3301,7 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
 			return -EAGAIN;
 	}
 out_free:
+	/* it's reportedly faster than delegating the null check to kfree() */
 	if (iovec)
 		kfree(iovec);
 	return ret;