lustre: 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>
Reviewed-on: http://review.whamcloud.com/8110
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3228


Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>