util: Make close_safe() reset fd to -1 even on close() failure

The "man 2 close":"Dealing with error returns from close()" says:

  "Retrying the close() after a failure return is the wrong thing to do"

We should not leave the fd there, attempting to close it again on next
close()/close_safe() may lead to accidentally closing something else.

It confirms with the kernel code where sys_close() removes fd from
fdtable in this stack:

  +-> sys_close
    +-> file_close_fd
      +-> file_close_fd_locked
        +-> rcu_assign_pointer(fdt->fd[fd], NULL)

If there was an fd this stack is always reached and fd is always
removed.

Let's replace the fd with -1 after close no matter what.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

criu/util.c

@@ -222,10 +222,9 @@ int close_safe(int *fd)
 
 	if (*fd > -1) {
 		ret = close(*fd);
-		if (!ret)
-			*fd = -1;
-		else
-			pr_perror("Unable to close fd %d", *fd);
+		if (ret)
+			pr_perror("Failed closing fd %d", *fd);
+		*fd = -1;
 	}
 
 	return ret;