LU-3659 llog: missed dt_try_as_dir() in llog_osd_dir_get()

LU-3659 llog: missed dt_try_as_dir() in llog_osd_dir_get()

dt_try_as_dir() should be called in llog_osd_dir_get() to make sure
the do_index_ops being initialized.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I879c47bb59fbd28f3351f61e59cff2a6666e7d7a
Reviewed-on: http://review.whamcloud.com/7267
Tested-by: Hudson
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>