selftests/bpf: skip sockmap in test_maps if kernel doesn't have support

Use recently introduced bpf_probe_map_type() to skip test_sockmap()
if map creation fails. The skipped test is indicated in the output.

Example:

test_sockmap SKIP (unsupported map type BPF_MAP_TYPE_SOCKMAP)
Fork 1024 tasks to 'test_update_delete'
...
test_sockmap SKIP (unsupported map type BPF_MAP_TYPE_SOCKMAP)
Fork 1024 tasks to 'test_update_delete'
...
test_maps: OK, 2 SKIPPED

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>