tg3: Expand 4g_overflow_test workaround to skb fragments of any size.

The current driver assumes that an skb fragment can only be upto jumbo
size. Presumably this was a fast-path optimization. This assumption is
no longer true as fragments can be upto 32k.

v2: Remove unnecessary parantheses per Eric Dumazet.

Cc: stable@vger.kernel.org
Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>