net: vlan: Replace BUG() with WARN_ON_ONCE() in vlan_dev_* stubs

net: vlan: Replace BUG() with WARN_ON_ONCE() in vlan_dev_* stubs

When CONFIG_VLAN_8021Q=n, a set of stub helpers are used, three of these
helpers use BUG() unconditionally.

This code should not be reached, as callers of these functions should
always check for is_vlan_dev() first, but the usage of BUG() is not
recommended, replace it with WARN_ON() instead.

Reviewed-by: Alex Lazar <alazar@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250616132626.1749331-3-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>