ethtool: tsinfo: fix uninitialized stats on the by-PHC path

ethtool: tsinfo: fix uninitialized stats on the by-PHC path

tsinfo_prepare_data() has two code paths: a "by-PHC" path for
user-specified hardware timestamping providers, and the old path.
Commit 89e281ebff72 ("ethtool: init tsinfo stats if requested") added
ethtool_stats_init() to mark stat slots as ETHTOOL_STAT_NOT_SET before
the driver callback populates them, but placed the call inside the
old-path block.

When commit b9e3f7dc9ed9 ("net: ethtool: tsinfo: Enhance tsinfo to
support several hwtstamp by net topology") added the by-PHC early
return, it landed above the stats initialization. On that path
the stats array retains the zero-fill from ethnl_init_reply_data()'s
zalloc. This leads to the reply including a stats nest with four
zero-valued attributes that should have been absent.

Reject GET requests for stats with HWTSTAMP_PROVIDER or dump.

Fixes: b9e3f7dc9ed9 ("net: ethtool: tsinfo: Enhance tsinfo to support several hwtstamp by net topology")
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260526153533.2779187-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>