bpf: make xdp sample variable names more meaningful

The naming choice of index is not terribly descriptive, and dropcnt is
in fact incorrect for xdp2. Pick better names for these: ipproto and
rxcnt.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>