LU-3544 llite: simplify dentry revalidate

LU-3544 llite: simplify dentry revalidate

Lustre client dentry validation is protected by LDLM lock, so
any time a dentry is found, it's valid and no need to revalidate
from MDS, and even it does, there is race that it may be
invalidated after revalidation is finished.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I7700cbaddc4ec08e12c9f7d8021783a6135dd35a
Reviewed-on: http://review.whamcloud.com/7475
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>