rxrpc: Request an ACK on impending Tx stall

Set the REQUEST-ACK flag on the DATA packet we're about to send if we're
about to stall transmission because the app layer isn't keeping up
supplying us with data to transmit.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20241204074710.990092-8-dhowells@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>