ppp: Fix false xmit recursion detect with two ppp devices

The global percpu variable ppp_xmit_recursion is used to detect the ppp
xmit recursion to avoid the deadlock, which is caused by one CPU tries to
lock the xmit lock twice. But it would report false recursion when one CPU
wants to send the skb from two different PPP devices, like one L2TP on the
PPPoE. It is a normal case actually.

Now use one percpu member of struct ppp instead of the gloable variable to
detect the xmit recursion of one ppp device.

Fixes: 55454a56

 ("ppp: avoid dealock on recursive xmit")
Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
Signed-off-by: Liu Jianying <jianying.liu@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>