rxrpc: Generate rtt_min

Generate rtt_min as this is required by RACK-TLP.

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/20241204074710.990092-27-dhowells@redhat.com


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