net: explain "protection types" for the instance lock

Try to define some terminology for which fields are protected
by which lock and how. Some fields are protected by both rtnl_lock
and instance lock which is hard to talk about without having
a "key phrase" to refer to a particular protection scheme.

"ops protected" fields are defined later in the series, one by one.

Add ASSERT_RTNL() to netdev_ops_assert_locked() for drivers
not other instance protection of ops. Hopefully it's not too
confusion that netdev_lock_ops() does not match the lock which
netdev_ops_assert_locked() will assert, exactly. The noun "ops"
is in a different place in the name, so I think it's acceptable...

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250324224537.248800-5-kuba@kernel.org


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