rxrpc: Use the core ICMP/ICMP6 parsers

Make rxrpc_encap_rcv_err() pass the ICMP/ICMP6 skbuff to ip_icmp_error() or
ipv6_icmp_error() as appropriate to do the parsing rather than trying to do
it in rxrpc.

This pushes an error report onto the UDP socket's error queue and calls
->sk_error_report() from which point rxrpc can pick it up.

It would be preferable to steal the packet directly from ip*_icmp_error()
rather than letting it get queued, but this is probably good enough.

Also note that __udp4_lib_err() calls sk_error_report() twice in some
cases.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org