rxrpc: Tidy up the ACK parsing a bit

rxrpc: Tidy up the ACK parsing a bit

Tidy up the ACK parsing in the following ways:

 (1) Put the serial number of the ACK packet into the rxrpc_ack_summary
     struct and access it from there whilst parsing an ACK.

 (2) Be consistent about using "if (summary.acked_serial)" rather than "if
     (summary.acked_serial != 0)".

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>