can: Propagate SO_PRIORITY of raw sockets to skbs

This allows controlling certain queueing disciplines by setting the
socket's SO_PRIORITY option.

For example, with the default pfifo_fast queueing discipline, which
provides three priorities, socket priority TC_PRIO_CONTROL means
higher than default and TC_PRIO_BULK means lower than default.

Signed-off-by: Rostislav Lisovy <lisovy@gmail.com>
Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>