page cache: Convert find_get_pages_contig to XArray

There's no direct replacement for radix_tree_for_each_contig()
in the XArray API as it's an unusual thing to do.  Instead,
open-code a loop using xas_next().  This removes the only user of
radix_tree_for_each_contig() so delete the iterator from the API and
the test suite code for it.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

.clang-format

@@ -323,7 +323,6 @@ ForEachMacros:
   - 'protocol_for_each_card'
   - 'protocol_for_each_dev'
   - 'queue_for_each_hw_ctx'
-  - 'radix_tree_for_each_contig'
   - 'radix_tree_for_each_slot'
   - 'radix_tree_for_each_tagged'
   - 'rbtree_postorder_for_each_entry_safe'