staging/lustre/llite: protect from accessing NULL lli_clob

Need to check file's lli_clob object before calling
lov_read_and_clear_async_rc().

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/23031
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8682


Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>