bpf: support packet data load instructions

bpf: support packet data load instructions

To fill the gap with linux kernel eBPF implementation,
add support for two non-generic instructions:
(BPF_ABS | <size> | BPF_LD) and (BPF_IND | <size> | BPF_LD)
which are used to access packet data.
These instructions can only be used when BPF context is a pointer
to 'struct rte_mbuf' (i.e: RTE_BPF_ARG_PTR_MBUF type).

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>