um: ubd: Add missing error check in start_io_thread()

The subsequent call to os_set_fd_block() overwrites the previous
return value. OR the two return values together to fix it.

Fixes: f88f0bd ("um: UBD Improvements")
Signed-off-by: Tiwei Bie <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Johannes Berg <[email protected]>

Author: btw616

arch/um/drivers/ubd_user.c

@@ -41,7 +41,7 @@ int start_io_thread(struct os_helper_thread **td_out, int *fd_out)
 	*fd_out = fds[1];
 
 	err = os_set_fd_block(*fd_out, 0);
-	err = os_set_fd_block(kernel_fd, 0);
+	err |= os_set_fd_block(kernel_fd, 0);
 	if (err) {
 		printk("start_io_thread - failed to set nonblocking I/O.\n");
 		goto out_close;