net: use correct this_cpu primitive in dev_recursion_level

net: use correct this_cpu primitive in dev_recursion_level

syzbot reports:
BUG: using __this_cpu_read() in preemptible code:
caller is dev_recursion_level include/linux/netdevice.h:3052 [inline]
 __this_cpu_preempt_check+0x246/0x270 lib/smp_processor_id.c:47
 dev_recursion_level include/linux/netdevice.h:3052 [inline]
 ip6_skb_dst_mtu include/net/ip6_route.h:245 [inline]

I erronously downgraded a this_cpu_read to __this_cpu_read when
moving dev_recursion_level() around.

Reported-by: syzbot+51471b4aae195285a4a3@syzkaller.appspotmail.com
Fixes: fc4d649ad01a ("net: place xmit recursion in softnet data")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>