ocfs2_dentry_attach_lock() can be executed in parallel threads against the
same dentry.  Make that race safe.  The race is like this:

            thread A                               thread B

(A1) enter ocfs2_dentry_attach_lock,
seeing dentry->d_fsdata is NULL,
and no alias found by
ocfs2_find_local_alias, so kmalloc
a new ocfs2_dentry_lock structure
to local variable "dl", dl1

               .....

                                    (B1) enter ocfs2_dentry_attach_lock,
                                    seeing dentry->d_fsdata is NULL,
                                    and no alias found by
                                    ocfs2_find_local_alias so kmalloc
                                    a new ocfs2_dentry_lock structure
                                    to local variable "dl", dl2.

                                                   ......

(A2) set dentry->d_fsdata with dl1,
call ocfs2_dentry_lock() and increase
dl1->dl_lockres.l_ro_hol...