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

d_delete() calls dentry_iput() after releasing the per dentry lock.  This
can race with __d_lookup and lead to situation where we can make dentry
negative with ref count > 1.  The following patch makes dentry_iput() to
hold per dentry lock till d_inode is NULL and dentry has been removed from
d_alias list.