btrfs: prevent RCU warning when dereferencing radix tree slot

Mark the dereference as protected by lock. Not doing so triggers
an RCU warning since the radix tree assumed that RCU is in use.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/tests/btrfs-tests.c

@@ -135,7 +135,7 @@ restart:
 	radix_tree_for_each_slot(slot, &fs_info->buffer_radix, &iter, 0) {
 		struct extent_buffer *eb;
 
-		eb = radix_tree_deref_slot(slot);
+		eb = radix_tree_deref_slot_protected(slot, &fs_info->buffer_lock);
 		if (!eb)
 			continue;
 		/* Shouldn't happen but that kind of thinking creates CVE's */