rxrpc: Define rxrpc_txbuf struct to carry data to be transmitted

Define a struct, rxrpc_txbuf, to carry data to be transmitted instead of a
socket buffer so that it can be placed onto multiple queues at once.  This
also allows the data buffer to be in the same allocation as the internal
data.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org