net: factor out skb_mac_gso_segment() from skb_gso_segment()

This function will be used in next GRE_GSO patch. This patch does
not change any functionality. It only exports skb_mac_gso_segment()
function.

[ Use skb_reset_mac_len() -DaveM ]

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>