memfd: Use radix_tree_deref_slot_protected to avoid the warning.

The commit 99b45e7a ("memfd: Fix locking when tagging pins")
introduces the following warning messages.

*WARNING: suspicious RCU usage in memfd_wait_for_pins*

It is because we still use radix_tree_deref_slot without read_rcu_lock.
We should use radix_tree_deref_slot_protected instead in the case.

Cc: stable@vger.kernel.org
Fixes: 99b45e7a ("memfd: Fix locking when tagging pins")
Signed-off-by: Nzhong jiang <zhongjiang@huawei.com>
Reviewed-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

mm/memfd.c

@@ -41,7 +41,7 @@ static void memfd_tag_pins(struct address_space *mapping)
 
 	xa_lock_irq(&mapping->i_pages);
 	radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) {
-		page = radix_tree_deref_slot(slot);
+		page = radix_tree_deref_slot_protected(slot, &mapping->i_pages.xa_lock);
 		if (!page || radix_tree_exception(page)) {
 			if (radix_tree_deref_retry(page)) {
 				slot = radix_tree_iter_retry(&iter);