ebpf, filter: do not convert skb->protocol to host endianess during runtime

Commit c2497395 ("bpf: allow BPF programs access 'protocol' and 'vlan_tci'
fields") has added support for accessing protocol, vlan_present and vlan_tci
into the skb offset map.

As referenced in the below discussion, accessing skb->protocol from an eBPF
program should be converted without handling endianess.

The reason for this is that an eBPF program could simply do a check more
naturally, by f.e. testing skb->protocol == htons(ETH_P_IP), where the LLVM
compiler resolves htons() against a constant automatically during compilation
time, as opposed to an otherwise needed run time conversion.

After all, the way of programming both from a user perspective differs quite
a lot, i.e. bpf_asm ["ld proto"] versus a C subset/LLVM.

Reference: https://patchwork.ozlabs.org/patch/450819/


Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>