afs: Use the per-peer app data provided by rxrpc

Make use of the per-peer application data that rxrpc now allows the
application to store on the rxrpc_peer struct to hold a back pointer to the
afs_server record that peer represents an endpoint for.

Then, when a call comes in to the AFS cache manager, this can be used to
map it to the correct server record rather than having to use a
UUID-to-server mapping table and having to do an additional lookup.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20250224234154.2014840-14-dhowells@redhat.com/ # v1
Link: https://lore.kernel.org/r/20250310094206.801057-10-dhowells@redhat.com/ # v4