bpf: testing: fix devmap tests

Apparently through one of my revisions of the initial patches
series I lost the devmap test. We can add more testing later but
for now lets fix the simple one we have.

Fixes: 546ac1ff "bpf: add devmap, a map for storing net device references"
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>