datapath: Fix recv path for CONFIG_PREEMPT_RCU.

datapath: Fix recv path for CONFIG_PREEMPT_RCU.

        In case CONFIG_PREEMPT_RCU, rcu grace period waits only for RCU
read-side critical sections that are delimited by rcu_read_lock() and
rcu_read_unlock(). internal_dev_xmit() is called in
rcu_read_lock_bh context. Therefore we need to explicitly take rcu
lock to prevent race with call_rcu() in PREEMPT_RCU case.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>