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/mincore.c

@@ -66,7 +66,7 @@ static unsigned char mincore_page(struct address_space *mapping, pgoff_t pgoff)
 		 * shmem/tmpfs may return swap: account for swapcache
 		 * page too.
 		 */
-		if (radix_tree_exceptional_entry(page)) {
+		if (xa_is_value(page)) {
 			swp_entry_t swp = radix_to_swp_entry(page);
 			page = find_get_page(swap_address_space(swp),
 					     swp_offset(swp));