Implement the peer RxRPC endpoint cache.  Only the primary cache is used.
This is indexed on the following details:

  - Network transport family - currently only AF_INET.
  - Network transport type - currently only UDP.
  - Peer network transport address.

We use the RCU read lock to handle non-creating lookups so that we can do
the look up from bottom half context in the sk_error_report handler.
Creating lookups are done under spinlock rather than mutex as they might be
set up due to an external stimulus if the local endpoint is a server.

Captured network error messages (ICMP) are handled with respect to this
struct and MTU size and RTT are cached here.
Signed-off-by: David Howells <dhowells@redhat.com>