net: ethtool: ioctl: concentrate the locking

net: ethtool: ioctl: concentrate the locking

Add another layer of helper functions to make upcoming locking
changes easier. Otherwise we'd need a pretty complex goto
structure. netdev instance lock is now taken slightly sooner
but that should not be an issue since rtnl_lock is already held,
anyway.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20260605002912.3456868-11-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>