sk_buff: allow segmenting based on frag sizes

This patch allows segmenting a skb based on its frags sizes instead of
based on a fixed value.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Tested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>