staging/lustre/llite: remove ll_d_root_ops

Mnt root dentry will never be revalidated, but its d_op->d_compare
will be called for its children, to simplify code, we use the same
ll_d_ops as normal dentries.
But its attribute may be invalid before access, this won't cause
any issue because it always exists, and the only operation depends
on its attribute is .permission, which has revalidated it in lustre
code.

So it's okay to remove ll_d_root_ops, and remove unnecessary checks
in lookup/revalidate/statahead.

Lustre-change: http://review.whamcloud.com/6797
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3486


Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Alexey Shvetsov <alexxy@gentoo.org>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>