rxrpc: Note cksum in txbuf

rxrpc: Note cksum in txbuf

Add a field to rxrpc_txbuf in which to store the checksum to go in the
header as this may get overwritten in the wire header struct when
transmitting as part of a jumbo packet.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org