xarray: Replace exceptional entries

Introduce xarray value entries and tagged pointers to replace radix
tree exceptional entries.  This is a slight change in encoding to allow
the use of an extra bit (we can now store BITS_PER_LONG - 1 bits in a
value entry).  It is also a change in emphasis; exceptional entries are
intimidating and different.  As the comment explains, you can choose
to store values or pointers in the xarray and they are both first-class
citizens.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Josef Bacik <jbacik@fb.com>

mm/madvise.c

@@ -251,7 +251,7 @@ static void force_shm_swapin_readahead(struct vm_area_struct *vma,
 		index = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
 
 		page = find_get_entry(mapping, index);
-		if (!radix_tree_exceptional_entry(page)) {
+		if (!xa_is_value(page)) {
 			if (page)
 				put_page(page);
 			continue;