ovpn: hash floated peer by transport identity only

ovpn: hash floated peer by transport identity only

The by_transp_addr table is keyed on the peer's remote transport
address, but the float rehash hashed bind->remote directly, while the
two other sites that touch the table build a clean key first:
ovpn_peer_add_mp() and the lookup in ovpn_peer_get_by_transp_addr()
both hash a sockaddr holding only family/address/port.

For a link-local IPv6 peer, bind->remote carries sin6_scope_id (set
from ipv6_iface_scope_id() when the endpoint is learned), and that
field is folded into the jhash() over sizeof(struct sockaddr_in6).
The lookup never sets sin6_scope_id, so after such a peer floats it is
rehashed into a scope_id-dependent bucket that lookups (scope_id 0)
never visit, making the peer unreachable through the by_transp_addr
fallback. ovpn_peer_transp_match() only compares address and port, so
the hash was keying on a field the match ignores.

sin6_scope_id must stay in bind->remote because the TX path uses it as
flowi6_oif, so it cannot just be cleared there. Instead build the hash
key from family/address/port only, exactly like ovpn_peer_add_mp() and
the lookup, so all three sites agree on the bucket.

Fixes: f0281c1d3732 ("ovpn: add support for updating local or remote UDP endpoint")
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>