rxrpc: Allow the app to store private data on peer structs

rxrpc: Allow the app to store private data on peer structs

Provide a way for the application (e.g. the afs filesystem) to store
private data on the rxrpc_peer structs for later retrieval via the call
object.

This will allow afs to store a pointer to the afs_server object on the
rxrpc_peer struct, thereby obviating the need for afs to keep lookup tables
by which it can associate an incoming call with server that transmitted it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
cc: netdev@vger.kernel.org
Link: https://lore.kernel.org/r/20250224234154.2014840-13-dhowells@redhat.com/
Link: https://lore.kernel.org/r/20250310094206.801057-9-dhowells@redhat.com/