rxrpc: Fix request for an ACK when cwnd is minimum

rxrpc_prepare_data_subpacket() sets the REQUEST-ACK flag on the outgoing
DATA packet under a number of circumstances, including, theoretically, when
the cwnd is at minimum (or less).  However, the minimum in this function is
hard-coded as 2, but the actual minimum is RXRPC_MIN_CWND (which is
currently 4) and so this never occurs.

Without this, we will miss the request of some ACKs, potentially leading to
a transmission stall until a timeout occurs on one side or the other that
leads to an ACK being generated.

Fix the function to use RXRPC_MIN_CWND rather than a hard-coded number.

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>