net: dsa: tag_8021q: avoid leaking ctx on dsa_tag_8021q_register() error path

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

net: dsa: tag_8021q: avoid leaking ctx on dsa_tag_8021q_register() error path

If dsa_tag_8021q_setup() fails, for example due to the inability of the

device to install a VLAN, the tag_8021q context of the switch will leak.

Make sure it is freed on the error path.