nfp: avoid buffer leak when representor is missing

When driver receives a muxed frame, but it can't find the representor
netdev it is destined to it will try to "drop" that frame, i.e. reuse
the buffer.  The issue is that the replacement buffer has already been
allocated at this point, and reusing the buffer from received frame
will leak it.  Change the code to put the new buffer on the ring
earlier and not reuse the old buffer (make the buffer parameter
to nfp_net_rx_drop() a NULL).

Fixes: 91bf82ca ("nfp: add support for tx/rx with metadata portid")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>