rxrpc: Fix use of wrong skb when comparing queued RESP challenge serial

In the Linux kernel, the following vulnerability has been resolved:

rxrpc: Fix use of wrong skb when comparing queued RESP challenge serial

In rxrpc_post_response(), the code should be comparing the challenge serial

number from the cached response before deciding to switch to a newer

response, but looks at the newer packet private data instead, rendering the

comparison always false.

Fix this by switching to look at the older packet.

Fix further[1] to substitute the new packet in place of the old one if

newer and also to release whichever we don't use.