nfp: reorganize nfp_net_rx() to get packet offsets early

Calculate packet offsets early in nfp_net_rx() so that we will be
able to use them in upcoming XDP handler.  While at it move relevant
variables into the loop scope.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>