can: dev: fix skb drop check

In the Linux kernel, the following vulnerability has been resolved:

can: dev: fix skb drop check

In commit a6d190f8c767 ("can: skb: drop tx skb if in listen only

mode") the priv->ctrlmode element is read even on virtual CAN

interfaces that do not create the struct can_priv at startup. This

out-of-bounds read may lead to CAN frame drops for virtual CAN

interfaces like vcan and vxcan.

This patch mainly reverts the original commit and adds a new helper

for CAN interface drivers that provide the required information in

struct can_priv.

[mkl: patch pch_can, too]