LU-5274 llite: fix ll_statahead_thread() problems on failure

LU-5274 llite: fix ll_statahead_thread() problems on failure

When ll_prep_md_op_data() fails, ll_statahead_thread() does not
release reference of sai and parent dentry. It does not wake
up parent thread either. This patch fixes these problems.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: Ibcc527d229e8a78710ff2263eb97969c7c46ecdb
Reviewed-on: http://review.whamcloud.com/10940
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>