rxrpc: Fix transport sockopts to get IPv4 errors on an IPv6 socket

It seems that enabling IPV6_RECVERR on an IPv6 socket doesn't also turn on
IP_RECVERR, so neither local errors nor ICMP-transported remote errors from
IPv4 peer addresses are returned to the AF_RXRPC protocol.

Make the sockopt setting code in rxrpc_open_socket() fall through from the
AF_INET6 case to the AF_INET case to turn on all the AF_INET options too in
the AF_INET6 case.

Fixes: f2aeed3a ("rxrpc: Fix error reception on AF_INET6 sockets")
Signed-off-by: David Howells <dhowells@redhat.com>