cxgb: fix T2 GSO

patch 7832ee03 in mainline.

The patch ensures that a GSO skb has enough headroom
to push an encapsulating cpl_tx_pkt_lso header.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>