net: lapb: Decrease the refcount of "struct lapb_cb" in lapb_device_event

In lapb_device_event, lapb_devtostruct is called to get a reference to
an object of "struct lapb_cb". lapb_devtostruct increases the refcount
of the object and returns a pointer to it. However, we didn't decrease
the refcount after we finished using the pointer. This patch fixes this
problem.

Fixes: a4989fa9

 ("net/lapb: support netdev events")
Cc: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Link: https://lore.kernel.org/r/20201231174331.64539-1-xie.he.0141@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>