io_uring: Clean up a false-positive warning from GCC 9.3.0

In io_recv(), if import_single_range() fails, the @flags variable is
uninitialized, then it will goto out_free.

After the goto, the compiler doesn't know that (ret < min_ret) is
always true, so it thinks the "if ((flags & MSG_WAITALL) ..."  path
could be taken.

The complaint comes from gcc-9 (Debian 9.3.0-22) 9.3.0:
```
  fs/io_uring.c:5238 io_recvfrom() error: uninitialized symbol 'flags'
```
Fix this by bypassing the @ret and @flags check when
import_single_range() fails.

Reasons:
 1. import_single_range() only returns -EFAULT when it fails.
 2. At that point, @flags is uninitialized and shouldn't be read.

Reported-by: kernel test robot <[email protected]>
Reported-by: Dan Carpenter <[email protected]>
Reported-by: "Chen, Rong A" <[email protected]>
Link: https://lore.gnuweeb.org/timl/[email protected]/
Cc: Pavel Begunkov <[email protected]>
Suggested-by: Ammar Faizi <[email protected]>
Fixes: 7297ce3 ("io_uring: improve send/recv error handling")
Signed-off-by: Alviro Iskandar Setiawan <[email protected]>
Signed-off-by: Ammar Faizi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: alviroiskandar

fs/io_uring.c

@@ -5228,17 +5228,16 @@ static int io_recv(struct io_kiocb *req, unsigned int issue_flags)
 		min_ret = iov_iter_count(&msg.msg_iter);
 
 	ret = sock_recvmsg(sock, &msg, flags);
-out_free:
 	if (ret < min_ret) {
 		if (ret == -EAGAIN && force_nonblock)
 			return -EAGAIN;
 		if (ret == -ERESTARTSYS)
 			ret = -EINTR;
 		req_set_fail(req);
 	} else if ((flags & MSG_WAITALL) && (msg.msg_flags & (MSG_TRUNC | MSG_CTRUNC))) {
+out_free:
 		req_set_fail(req);
 	}
-
 	__io_req_complete(req, issue_flags, ret, io_put_kbuf(req));
 	return 0;
 }