fs: prevent page refcount overflow in pipe_buf_get

Change pipe_buf_get() to return a bool indicating whether it succeeded
in raising the refcount of the page (if the thing in the pipe is a page).
This removes another mechanism for overflowing the page refcount.  All
callers converted to handle a failure.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
{
	spin_lock(&fiq->waitq.lock);
	if (test_bit(FR_FINISHED, &req->flags)) {
		spin_unlock(&fiq->waitq.lock);
		return;
	}
	if (list_empty(&req->intr_entry)) {
		list_add_tail(&req->intr_entry, &fiq->interrupts);
		wake_up_locked(&fiq->waitq);
	}
	spin_unlock(&fiq->waitq.lock);
	kill_fasync(&fiq->fasync, SIGIO, POLL_IN);
}

static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
{
	spin_lock(&fiq->waitq.lock);
	if (test_bit(FR_FINISHED, &req->flags)) {
		spin_unlock(&fiq->waitq.lock);
		return;
	}
	if (list_empty(&req->intr_entry)) {
		list_add_tail(&req->intr_entry, &fiq->interrupts);
		wake_up_locked(&fiq->waitq);
	}
	spin_unlock(&fiq->waitq.lock);
	kill_fasync(&fiq->fasync, SIGIO, POLL_IN);
}
