devicemapper: fix zero-sized field access
Fixes: #15279
Due to
https://github.com/golang/go/commit/
7904946eeb35faece61bbf6f5b3cc8be2f519c17
the devices field is dropped.
This solution works on go1.4 and go1.5
Signed-off-by: Vincent Batts <vbatts@redhat.com>
Gbp-Pq: Name 15404.patch