ipv6: old_dport should be a __be16 in __ip6_datagram_connect()

Fixes: 2f987a76 ("net: ipv6: keep sk status consistent after datagram connect failure")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>