selftests/bpf, flow_dissector: Close TAP device FD after the test

test_flow_dissector leaves a TAP device after it's finished, potentially
interfering with other tests that will run after it. Fix it by closing the
TAP descriptor on cleanup.

Fixes: 0905beec9f52 ("selftests/bpf: run flow dissector tests in skb-less mode")
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200531082846.2117903-11-jakub@cloudflare.com

tools/testing/selftests/bpf/prog_tests/flow_dissector.c

@@ -524,6 +524,7 @@ void test_flow_dissector(void)
 		CHECK_ATTR(err, tests[i].name, "bpf_map_delete_elem %d\n", err);
 	}
 
+	close(tap_fd);
 	bpf_prog_detach(prog_fd, BPF_FLOW_DISSECTOR);
 	bpf_object__close(obj);
 }