f_flower: Implement gtp options support

f_flower: Implement gtp options support

Add support for parsing TCA_FLOWER_KEY_ENC_OPTS_GTP.
Options are as follows: PDU_TYPE:QFI where each
option is represented as 8-bit hexadecimal value.

e.g.
  # ip link add gtp_dev type gtp role sgsn
  # tc qdisc add dev gtp_dev ingress
  # tc filter add dev gtp_dev protocol ip parent ffff: \
      flower \
        enc_key_id 11 \
        gtp_opts 1:8/ff:ff \
      action mirred egress redirect dev eth0

Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: David Ahern <dsahern@kernel.org>