slcan: ldisc generated skbs are received in softirq context

As this discussion pointed out

http://marc.info/?l=linux-netdev&m=131257225602375



netdevices that are based on serial line disciplines should use netif_rx_ni()
when pushing received socketbuffers into the netdev rx queue.

Following commit 61485160 ("slip: fix NOHZ
local_softirq_pending 08 warning") this patch updates the slcan driver
accordingly.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
CC: Matvejchikov Ilya <matvejchikov@gmail.com>
CC: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>