selftests/bpf: Replace bpf_trace_printk in tunnel kernel code

Replace bpf_trace_printk with bpf_printk in test_tunnel_kern.c.
function bpf_printk is more easier and useful than bpf_trace_printk.

Signed-off-by: Kaixi Fan <fankaixi.li@bytedance.com>
Link: https://lore.kernel.org/r/20220430074844.69214-4-fankaixi.li@bytedance.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>