btrfs: tree-log.c: Wrong printk information about namelen

In verify_dir_item, it wants to printk name_len of dir_item but
printk data_len acutally.

Fix it by calling btrfs_dir_name_len instead of btrfs_dir_data_len.

Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>