fix a fencepost error in pipe_advance()

The logics in pipe_advance() used to release all buffers past the new
position failed in cases when the number of buffers to release was equal
to pipe->buffers.  If that happened, none of them had been released,
leaving pipe full.  Worse, it was trivial to trigger and we end up with
pipe full of uninitialized pages.  IOW, it's an infoleak.

Cc: stable@vger.kernel.org # v4.9
Reported-by: "Alan J. Wylie" <alan@wylie.me.uk>
Tested-by: "Alan J. Wylie" <alan@wylie.me.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
static bool sanity(const struct iov_iter *i)
{
	struct pipe_inode_info *pipe = i->pipe;
	int idx = i->idx;
	int next = pipe->curbuf + pipe->nrbufs;
	if (i->iov_offset) {
		struct pipe_buffer *p;
		if (unlikely(!pipe->nrbufs))
			goto Bad;	// pipe must be non-empty
		if (unlikely(idx != ((next - 1) & (pipe->buffers - 1))))
			goto Bad;	// must be at the last buffer...

		p = &pipe->bufs[idx];
		if (unlikely(p->offset + p->len != i->iov_offset))
			goto Bad;	// ... at the end of segment
	} else {
		if (idx != (next & (pipe->buffers - 1)))
			goto Bad;	// must be right after the last buffer
	}
	return true;
Bad:
	printk(KERN_ERR "idx = %d, offset = %zd\n", i->idx, i->iov_offset);
	printk(KERN_ERR "curbuf = %d, nrbufs = %d, buffers = %d\n",
			pipe->curbuf, pipe->nrbufs, pipe->buffers);
	for (idx = 0; idx < pipe->buffers; idx++)
		printk(KERN_ERR "[%p %p %d %d]\n",
			pipe->bufs[idx].ops,
			pipe->bufs[idx].page,
			pipe->bufs[idx].offset,
			pipe->bufs[idx].len);
	WARN_ON(1);
	return false;
}

static bool sanity(const struct iov_iter *i)
{
	struct pipe_inode_info *pipe = i->pipe;
	int idx = i->idx;
	int next = pipe->curbuf + pipe->nrbufs;
	if (i->iov_offset) {
		struct pipe_buffer *p;
		if (unlikely(!pipe->nrbufs))
			goto Bad;	// pipe must be non-empty
		if (unlikely(idx != ((next - 1) & (pipe->buffers - 1))))
			goto Bad;	// must be at the last buffer...

		p = &pipe->bufs[idx];
		if (unlikely(p->offset + p->len != i->iov_offset))
			goto Bad;	// ... at the end of segment
	} else {
		if (idx != (next & (pipe->buffers - 1)))
			goto Bad;	// must be right after the last buffer
	}
	return true;
Bad:
	printk(KERN_ERR "idx = %d, offset = %zd\n", i->idx, i->iov_offset);
	printk(KERN_ERR "curbuf = %d, nrbufs = %d, buffers = %d\n",
			pipe->curbuf, pipe->nrbufs, pipe->buffers);
	for (idx = 0; idx < pipe->buffers; idx++)
		printk(KERN_ERR "[%p %p %d %d]\n",
			pipe->bufs[idx].ops,
			pipe->bufs[idx].page,
			pipe->bufs[idx].offset,
			pipe->bufs[idx].len);
	WARN_ON(1);
	return false;
}
