rxrpc: Show consumed and freed packets as non-dropped in dropwatch

Set a reason when freeing a packet that has been consumed such that
dropwatch doesn't complain that it has been dropped.

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