LU-3373 llite: instantiate negative dentry

In the atomic_open callback. We should instantiate
negative dentry. Else will got sanity:183 failed.

Signed-off-by: yang sheng <yang.sheng@intel.com>
Change-Id: I83bc4a8995dfb813a381e910cc94cef6fe3721c3
Reviewed-on: http://review.whamcloud.com/8110
Tested-by: Jenkins
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>