net: stmmac: fix jumbo frame sending with non-linear skbs

When sending non-linear skbs with jumbo frames, we set up the non-paged
data and mark that as a last segment, although the paged fragments are
also prepared. This will stall the TX queue and trigger a watchdog warning
(a simple reproducer is to run an iperf client mode TCP test with a large
MTU - networking fails instantly).

Fix by checking if the skb is non-linear.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Acked-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>