rxrpc: Clean up Tx header flags generation handling

Clean up the generation of the header flags when building packet headers
for transmission:

 (1) Assemble the flags in a local variable rather than in the txb->flags.

 (2) Do the flags masking and JUMBO-PACKET setting in one bit of code for
     both the main header and the jumbo headers.

 (3) Generate the REQUEST-ACK flag afresh each time.  There's a possibility
     we might want to do jumbo retransmission packets in future.

 (4) Pass the local flags variable to the rxrpc_tx_data tracepoint rather
     than the combination of the txb flags and the wire header flags (the
     latter belong only to the first subpacket).

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/20241204074710.990092-5-dhowells@redhat.com


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