net/sched: act_mirred: refactor the handle of xmit

[ Upstream commit fa6d639930ee5cd3f932cc314f3407f07a06582d ]

This one is prepare for the next patch.

Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ skulkarni: Adjusted patch for file 'sch_generic.h' wrt the mainline commit ]
Stable-dep-of: ca22da2fbd69 ("act_mirred: use the backlog for nested calls to mirred ingress")
Signed-off-by: Shubham Kulkarni <skulkarni@mvista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>