dax: Flush partial PMDs correctly

The radix tree would rewind the index in an iterator to the lowest index
of a multi-slot entry.  The XArray iterators instead leave the index
unchanged, but I overlooked that when converting DAX from the radix tree
to the XArray.  Adjust the index that we use for flushing to the start
of the PMD range.

Fixes: c1901cd3

 ("page cache: Convert find_get_entries_tag to XArray")
Cc: <stable@vger.kernel.org>
Reported-by: Piotr Balcer <piotr.balcer@intel.com>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>