bpf: update BPF_PROG_LOAD decoding

bpf: update BPF_PROG_LOAD decoding

Implement decoding of union bpf_attr.prog_flags field for BPF_PROG_LOAD
command introduced by linux kernel commit v4.12-rc2~34^2~29^2~2.

* configure.ac: Check for prog_flags member of union bpf_attr
instead of kern_version.
* xlat/bpf_prog_flags.in: New file.
* bpf.c: Include "xlat/bpf_prog_flags.h".
(decode_BPF_PROG_LOAD): Add prog_flags field to the structure, print it.
* tests/bpf.c: Update macro guards of BPF_PROG_LOAD decoder test.
(init_BPF_PROG_LOAD_first, print_BPF_PROG_LOAD_attr): Update expected
output.
(init_BPF_PROG_LOAD_attr): Initialize prog_flags field, update offset.