bcachefs: thread_with_stdio: fix spinning instead of exiting

bch2_stdio_redirect_vprintf() was missing a check for stdio->done, i.e.
exiting.

This caused the thread attempting to print to spin, and since it was
being called from the kthread ran by thread_with_stdio, the userspace
side hung as well.

Change it to return -EPIPE - i.e. writing to a pipe that's been closed.

Reported-by: Jan Solanti <jhs@psonet.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/thread_with_file.c

@@ -455,8 +455,10 @@ ssize_t bch2_stdio_redirect_vprintf(struct stdio_redirect *stdio, bool nonblocki
 	struct stdio_buf *buf = &stdio->output;
 	unsigned long flags;
 	ssize_t ret;
-
 again:
+	if (stdio->done)
+		return -EPIPE;
+
 	spin_lock_irqsave(&buf->lock, flags);
 	ret = bch2_darray_vprintf(&buf->buf, GFP_NOWAIT, fmt, args);
 	spin_unlock_irqrestore(&buf->lock, flags);