LU-2275 mdt: Avoid setting positive dispositions too early

LU-2275 mdt: Avoid setting positive dispositions too early

mdt_finish_open and mdt_open_by_fid_lock set DISP_LOOKUP_POS
and DISP_OPEN_OPEN a bit too eagerly only to have later code add
negative disposition and a lookup error, confusing a client
with such self-contradicting replies in the end.

This patch only sets positive dispositions when we are sure
not to get them overruled.

Change-Id: I3389f1d59b21f4a9e4c16ca75b398785ea6e130c
Signed-off-by: Oleg Drokin <green@whamcloud.com
Reviewed-on: http://review.whamcloud.com/4459
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>