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

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

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