1) If tcp_{sendmsg,sendpage} tacks on more data to an
   existing SKB, this can make tso_factor inaccurate.
   Invalidate it, which forces it to be recalculated,
   by simply setting it to zero.
2) __tcp_trim_head() changes skb->len thus we need
   to recalculate tso_factor
3) BUG check that tcp_retrans_try_collapse() does not
   try to collapse packets with non-1 tso_factor
4) The Solaris FIN workaround in tcp_retransmit_skb()
   changes packet size, need to fixup tso_factor
Signed-off-by: David S. Miller <davem@davemloft.net>