ocfs2: Hold ip_lock when set/clear flags for indexed dir.

When we set/clear the dyn_features for an inode we hold the ip_lock.
So do it when we set/clear OCFS2_INDEXED_DIR_FL also.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>