samples: bpf: relax test_maps check

hash map is unordered, so get_next_key() iterator shouldn't
rely on particular order of elements. So relax this test.

Fixes: ffb65f27a155 ("bpf: add a testsuite for eBPF maps")
Reported-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>