rxrpc: Make rxrpc_send_packet() take a connection not a transport

Make rxrpc_send_packet() take a connection not a transport as part of the
phasing out of the rxrpc_transport struct.

Whilst we're at it, rename the function to rxrpc_send_data_packet() to
differentiate it from the other packet sending functions.

Signed-off-by: David Howells <dhowells@redhat.com>