LU-36 mdd: cache lu_attr to avoid duplicate fetch

LU-36 mdd: cache lu_attr to avoid duplicate fetch

- we called mdd_la_get for so many times in MDD, it's not good for
  performance of high contention pdir operations.
- mdd_attr_check is not strictly correct under pdir operations,
  there is still chance to decrease ctime because we don't have
  exclusive lock on parent while checking ctime
- mdd_attr_check_set_internal and mdd_attr_check_set_internal_locked
  can be merged into one function: mdd_update_time, because set
  cmtime is the only use-case of them.

Change-Id: I59500835473ae6d42aa9bd2dcf9be644b7b7ec16
Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-on: http://review.whamcloud.com/6897
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>