tg3: Clear NETIF_F_TSO6 flag before doing software GSO

Commit d3f6f3a1 ("tg3: Prevent page
allocation failure during TSO workaround") modified driver logic
to use tg3_tso_bug() for any TSO fragment that hits hardware bug
conditions thus the patch increased the scope of work for tg3_tso_bug()
to cover devices that support NETIF_F_TSO6 as well. Prior to the
patch, tg3_tso_bug() would only be used on devices supporting
NETIF_F_TSO.

A regression was introduced for IPv6 packets requiring the workaround.
To properly perform GSO on SKBs with TCPV6 gso_type, we need to call
skb_gso_segment() with NETIF_F_TSO6 feature flag cleared, or the
function will return NULL and cause a kernel oops as tg3 is not handling
a NULL return value. This patch fixes the problem.

Signed-off-by: Prashant Sreedharan <prashant@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>