LU-3473 llite: speedup in unlink/rmdir

LU-3473 llite: speedup in unlink/rmdir

Assume dchild argument is fully initialized in ->unlink and
->rmdir callbacks, so additional lookup for ELC is not needed.

Reviewed-by: Alexander Boyko <Alexander_Boyko@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>
Xyratex-bug-id: MRP-1027
Signed-off-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Change-Id: I984cc8260f8fb5be1f09938b2b38399636c2acd9
Reviewed-on: http://review.whamcloud.com/6648
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>