rxrpc: Change module filename to rxrpc.ko

Change module filename from af-rxrpc.ko to rxrpc.ko so as to be consistent
with the other protocol drivers.

Also adjust the documentation to reflect this.

Further, there is no longer a standalone rxkad module, as it has been
merged into the rxrpc core, so get rid of references to that.

Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>