slip/slcan: added locking in wakeup function

The locking is needed, since the the internal buffer for the CAN frames is
changed during the wakeup call. This could cause buffer inconsistencies
under high loads, especially for the outgoing short CAN packet skbuffs.

The needed locks led to deadlocks before commit
"5ede5253 tty: Remove extra wakeup from pty
write() path", which removed the direct callback to the wakeup function from the
tty layer.

As slcan.c is based on slip.c the issue in the original code is fixed, too.

Signed-off-by: Andre Naujoks <nautsch2@gmail.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>