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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Kaixi Fan