net: netlink: fix sending unassigned nsid after assigned one

net: netlink: fix sending unassigned nsid after assigned one

If the current skb is not shared, it is re-used directly for all the
sockets subscribed to the notification.  If we have remote all-nsid
socket receiving a message first, then the 'nsid_is_set' will be
set to 'true'.  If the nsid is NOT_ASSIGNED for the next socket in
the list, the 'nsid_is_set' will remain 'true' and the negative value
is be delivered to the user space.  All subsequent nsid values will be
delivered as well, since there is no code path that sets the flag
back to 'false'.

Fix that by always dropping the flag to 'false' first.

Fixes: 7212462fa6fd ("netlink: don't send unknown nsid")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://patch.msgid.link/20260520172317.175168-2-i.maximets@ovn.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>