selftests/bpf: speedup test_maps

test_hashmap_walk takes very long time on debug kernel with kasan on.
Reduce the number of iterations in this test without sacrificing
test coverage.
Also add printfs as progress indicator.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>