net: ethtool: fix leaking netdev ref if ethnl_default_parse() failed

Ido spotted that I made a mistake in commit under Fixes,
ethnl_default_parse() may acquire a dev reference even when it returns
an error. This may have been driven by the code structure in dumps
(which unconditionally release dev before handling errors), but it's
too much of a trap. Functions should undo what they did before returning
an error, rather than expecting caller to clean up.

Rather than fixing ethnl_default_set_doit() directly make
ethnl_default_parse() clean up errors.

Reported-by: Ido Schimmel <idosch@idosch.org>
Link: https://lore.kernel.org/aGEPszpq9eojNF4Y@shredder


Fixes: 963781bd ("net: ethtool: call .parse_request for SET handlers")
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250630154053.1074664-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>