net,RT:REmove preemption disabling in netif_rx()

net,RT:REmove preemption disabling in netif_rx()

1)enqueue_to_backlog() (called from netif_rx) should be
  bind to a particluar CPU. This can be achieved by
  disabling migration. No need to disable preemption

2)Fixes crash "BUG: scheduling while atomic: ksoftirqd"
  in case of RT.
  If preemption is disabled, enqueue_to_backog() is called
  in atomic context. And if backlog exceeds its count,
  kfree_skb() is called. But in RT, kfree_skb() might
  gets scheduled out, so it expects non atomic context.

3)When CONFIG_PREEMPT_RT_FULL is not defined,
 migrate_enable(), migrate_disable() maps to
 preempt_enable() and preempt_disable(), so no
 change in functionality in case of non-RT.

-Replace preempt_enable(), preempt_disable() with
 migrate_enable(), migrate_disable() respectively
-Replace get_cpu(), put_cpu() with get_cpu_light(),
 put_cpu_light() respectively

Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Acked-by: Rajan Srivastava <Rajan.Srivastava@freescale.com>
Cc: <rostedt@goodmis.orgn>
Link: http://lkml.kernel.org/r/1337227511-2271-1-git-send-email-Priyanka.Jain@freescale.com
Cc: stable-rt@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>