rxrpc: Add a tracepoint to show variables pertinent to jumbo packet size

rxrpc: Add a tracepoint to show variables pertinent to jumbo packet size

Add a tracepoint to be called right before packets are transmitted for the
first time that shows variable values that are pertinent to how many
subpackets will be added to a jumbo DATA packet.

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/20241204074710.990092-13-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>