rxrpc: Fix to request an ack if window is limited

rxrpc: Fix to request an ack if window is limited

Peers may only send immediate acks for every 2 UDP packets received.
When sending a jumbogram, it is important to check that there is
sufficient window space to send another same sized jumbogram following
the current one, and request an ack if there isn't.  Failure to do so may
cause the call to stall waiting for an ack until the resend timer fires.

Where jumbograms are in use this causes a very significant drop in
performance.

Fixes: fe24a5494390 ("rxrpc: Send jumbo DATA packets")
Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260408121252.2249051-10-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>