rxrpc: Add a reason indicator to the tx_data tracepoint

Add an indicator to the rxrpc_tx_data tracepoint to indicate what triggered
the transmission of a particular packet.  At this point, it's only normal
transmission and retransmission, plus the tracepoint is also used to record
loss injection, but in a future patch, TLP-induced (re-)transmission will
also be a thing.

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