net: ethtool: serialize broadcast notification sequence allocation

ethnl_bcast_seq is a global counter stamped into the nlmsg_seq field
of every multicast notification, allowing userspace to detect dropped
messages. Today the ordering is achieved by using rtnl_lock().

Moving forward we will want ethtool ops to run under just the netdev
instance lock so to establish ordering we need a separate lock
for notifications. With the netdev instance locks operations on
different devices may bypass each other but the expectation is
that it should not matter. What we need to prevent is:
 - notification IDs getting out of order
 - operations on one device getting out of order

For simplicity defer allocating the ID of the notification right
before the notification is delivered. This removes the need for
special handling in ethnl_rss_create_send_ntf().

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/20260605002912.3456868-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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