LU-2801 mdt: checking remote object in mdt_swap_layouts

It should use mdt_remote_object to check remote object in
mdt_swap_layouts, instead of mdt_object_exists < 0.

Send swap layout req to the right MDT.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Ia24635e16cb67e5683abc9797ec6c9e61f648c62
Reviewed-on: http://review.whamcloud.com/5332
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>