filesystem-dax: Fix dax_layout_busy_page() livelock

In the presence of multi-order entries the typical
pagevec_lookup_entries() pattern may loop forever:

	while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
				min(end - index, (pgoff_t)PAGEVEC_SIZE),
				indices)) {
		...
		for (i = 0; i < pagevec_count(&pvec); i++) {
			index = indices[i];
			...
		}
		index++; /* BUG */
	}

The loop updates 'index' for each index found and then increments to the
next possible page to continue the lookup. However, if the last entry in
the pagevec is multi-order then the next possible page index is more
than 1 page away. Fix this locally for the filesystem-dax case by
checking for dax-multi-order entries. Going forward new users of
multi-order entries need to be similarly careful, or we need a generic
way to report the page increment in the radix iterator.

Fixes: 5fac7408 ("mm, fs, dax: handle layout changes to pinned dax...")
Cc: <stable@vger.kernel.org>
Cc: Ross Zwisler <zwisler@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>