backports: remove usage of skbuff->xmit_more

backports: remove usage of skbuff->xmit_more

This is a revet of these two upstream commits:
commit c1ebf46c1f72fe542853fc00f059a7d15259379d
Author: David S. Miller <davem@davemloft.net>
Date:   Fri Aug 22 17:24:49 2014 -0700

    igb: Support netdev_ops->ndo_xmit_flush()

commit 0b725a2ca61bedc33a2a63d0451d528b268cf975
Author: David S. Miller <davem@davemloft.net>
Date:   Mon Aug 25 15:51:53 2014 -0700

    net: Remove ndo_xmit_flush netdev operation, use signalling instead.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>