LU-5144 mdt: rename vs link deadlock

LU-5144 mdt: rename vs link deadlock

it may happen that tgt child for rename is a directory, which is used
as tgt dir for link in parallel, as the result rename locks src child,
tgt child what may deadlock with tgt dir, src file locking of link.

As not dir cannot replace dir, check it before locking the tgt child.

Signed-off-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Xyratex-bug-id: MRP-1921
Change-Id: If8b91d27e4d978853e4b8fbb025667d22c9d2f02
Reviewed-on: http://review.whamcloud.com/10570
Tested-by: Jenkins
Tested-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>