net: veth: fix ethtool stats reporting

Fix a possible misalignment between page_pool stats and tx xdp_stats
reported in veth_get_ethtool_stats routine.
The issue can be reproduced configuring the veth pair with the
following tx/rx queues:

$ip link add v0 numtxqueues 2 numrxqueues 4 type veth peer name v1 \
 numtxqueues 1 numrxqueues 1

and loading a simple XDP program on v0 that just returns XDP_PASS.
In this case on v0 the page_pool stats overwrites tx xdp_stats for queue 1.
Fix the issue incrementing pp_idx of dev->real_num_tx_queues * VETH_TQ_STATS_LEN
since we always report xdp_stats for all tx queues in ethtool.

Fixes: 4fc41805

 ("net: veth: add page_pool stats")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/c5b5d0485016836448453f12846c7c4ab75b094a.1700593593.git.lorenzo@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>