btrfs: tree-checker: check for BTRFS_BLOCK_FLAG_FULL_BACKREF being set improperly

We need to validate that a data extent item does not have the
FULL_BACKREF flag set on its flags.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-checker.c

@@ -1290,6 +1290,11 @@ static int check_extent_item(struct extent_buffer *leaf,
 				   key->offset, fs_info->sectorsize);
 			return -EUCLEAN;
 		}
+		if (unlikely(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF)) {
+			extent_err(leaf, slot,
+			"invalid extent flag, data has full backref set");
+			return -EUCLEAN;
+		}
 	}
 	ptr = (unsigned long)(struct btrfs_extent_item *)(ei + 1);