rxrpc: Clean up the resend algorithm

Clean up the DATA packet resending algorithm to retransmit packets as we
come across them whilst walking the transmission buffer rather than queuing
them for retransmission at the end.  This can be done as ACK parsing - and
thus the discarding of successful packets - is now done in the same thread
rather than separately in softirq context and a locked section is no longer
required.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org