can: convert protocol handling to RCU

can: convert protocol handling to RCU

This patch removes spin_locks at CAN socket creation time by using RCU.

Inspired by the discussion with Kurt van Dijck and Eric Dumazet the RCU code
was partly derived from af_phonet.c

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Reviewed-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Signed-off-by: David S. Miller <davem@davemloft.net>
git-svn-id: svn://svn.berlios.de//socketcan/trunk@1238 030b6a49-0b11-0410-94ab-b0dab22257f2