net: ethtool: fix NULL pointer dereference in stats_prepare_data()

In the following call path:

ethnl_default_dumpit
-> ethnl_default_dump_one
   -> ctx->ops->prepare_data
      -> stats_prepare_data

struct genl_info *info will be passed as NULL, and stats_prepare_data()
dereferences it while getting the extended ack pointer.

To avoid that, just set the extack to NULL if "info" is NULL, since the
netlink extack handling messages know how to deal with that.

The pattern "info ? info->extack : NULL" is present in quite a few other
"prepare_data" implementations, so it's clear that it's a more general
problem to be dealt with at a higher level, but the code should have at
least adhered to the current conventions to avoid the NULL dereference.

Fixes: 04692c90

 ("net: ethtool: netlink: retrieve stats from multiple sources (eMAC, pMAC)")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>