rtnetlink: Fix handling of disabled L3 stats in RTM_GETSTATS replies

rtnetlink: Fix handling of disabled L3 stats in RTM_GETSTATS replies

When L3 stats are disabled, rtnl_offload_xstats_get_size_stats() returns
size of 0, which is supposed to be an indication that the corresponding
attribute should not be emitted. However, instead, the current code
reserves a 0-byte attribute.

The reason this does not show up as a citation on a kasan kernel is that
netdev_offload_xstats_get(), which is supposed to fill in the data, never
ends up getting called, because rtnl_offload_xstats_get_stats() notices
that the stats are not actually used and skips the call.

Thus a zero-length IFLA_OFFLOAD_XSTATS_L3_STATS attribute ends up in a
response, confusing the userspace.

Fix by skipping the L3-stats related block in rtnl_offload_xstats_fill().

Fixes: 8bed40493a1e ("net: rtnetlink: Add UAPI for obtaining L3 offload xstats")
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/591b58e7623edc3eb66dd1fcfa8c8f133d090974.1649794741.git.petrm@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>