mm: Convert __do_page_cache_readahead to XArray

This one is trivial.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

mm/readahead.c

@@ -176,9 +176,7 @@ unsigned int __do_page_cache_readahead(struct address_space *mapping,
 		if (page_offset > end_index)
 			break;
 
-		rcu_read_lock();
-		page = radix_tree_lookup(&mapping->i_pages, page_offset);
-		rcu_read_unlock();
+		page = xa_load(&mapping->i_pages, page_offset);
 		if (page && !xa_is_value(page)) {
 			/*
 			 * Page already present?  Kick off the current batch of