bpf: print xlated_prog_insns only when xlated_prog_len had not been 0

bpf: print xlated_prog_insns only when xlated_prog_len had not been 0

If xlated_prog_len is zero on entering syscall and non-zero on exiting,
the program itself is not written into xlated_prog_insns.

* bpf.c (print_ebpf_prog): Add decode argument, print the address only
if decode is false.
(BEGIN_BPF_CMD_DECODER(BPF_PROG_LOAD)): Specify decode == true argument
to print_ebpf_prog.
(print_bpf_prog_info): Specify saved xlated_prog_len as decode argument
to print_ebpf_prog.

Co-Authored-by: Dmitry V. Levin <ldv@altlinux.org>