LU-4367 llite: Make revalidate return 0 for opens

This is important so that we can properly engage opencache when needed.

This also reshuffles some other logic in ll_revalidate_dentry to
more properly handle intermediate component revalidate logic.

Change-Id: Ib4f272155096d33fe3f45b781e21e5bd4f44ff58
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/11062
Tested-by: Jenkins
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>