rxrpc: Prepare to be able to send jumbo DATA packets

rxrpc: Prepare to be able to send jumbo DATA packets

Prepare to be able to send jumbo DATA packets if the we decide to, but
don't enable that yet.  This will allow larger chunks of data to be sent
without reducing the retryability as the subpackets in a jumbo packet can
also be retransmitted individually.

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/20241204074710.990092-12-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>