net: bcmgenet: Be drop monitor friendly while re-allocating headroom

During bcmgenet_put_tx_csum() make sure we differentiate a SKB
headroom re-allocation failure from the normal swap and replace
path.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>