rxrpc: Fix the calculation and use of RTO

Make the following changes to the calculation and use of RTO:

 (1) Fix rxrpc_resend() to use the backed-off RTO value obtained by calling
     rxrpc_get_rto_backoff() rather than extracting the value itself.
     Without this, it may retransmit packets too early.

 (2) The RTO value being similar to the RTT causes a lot of extraneous
     resends because the RTT doesn't end up taking account of clearing out
     of the receive queue on the server.  Worse, responses to PING-ACKs are
     made as fast as possible and so are less than the DATA-requested-ACK
     RTT and so skew the RTT down.

     Fix this by putting a lower bound on the RTO by adding 100ms to it and
     limiting the lower end to 200ms.

Fixes: c410bf01 ("rxrpc: Fix the excessive initial retransmission timeout")
Fixes: 37473e41 ("rxrpc: Clean up the resend algorithm")
Signed-off-by: David Howells <dhowells@redhat.com>
Suggested-by: Simon Wilkinson <sxw@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>