From: Maneesh Soni <maneesh@in.ibm.com>

o The following patch fixes is_subdir() races with d_move. Due to concurrent
  d_move, in is_subdir() we can end up accessing freed d_parent pointer in
  case of pre-emptible kernel. To avoid this we can use rcu_read_lock() and
  rcu_read_unlock().

o This also fixes the seqlock uses in is_subdir() as we need to restart the
  the inner loop with the origianl new_dentry passed to the routine in case
  of any rename occured while we are traversing d_parent links.