rxrpc: Use rxrpc_extract_addr_from_skb() rather than doing this manually

There are two places that want to transmit a packet in response to one just
received and manually pick the address to reply to out of the sk_buff.
Make them use rxrpc_extract_addr_from_skb() instead so that IPv6 is handled
automatically.

Signed-off-by: David Howells <dhowells@redhat.com>