net: vlan: Make is_vlan_dev() a stub when VLAN is not configured

net: vlan: Make is_vlan_dev() a stub when VLAN is not configured

Add a stub implementation of is_vlan_dev() that returns false when
VLAN support is not compiled in (CONFIG_VLAN_8021Q=n).

This allows us to compile-out VLAN-dependent dead code when it is not
needed.

This also resolves the following compilation error when:
* CONFIG_VLAN_8021Q=n
* CONFIG_OBJTOOL=y
* CONFIG_OBJTOOL_WERROR=y

drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.o: error: objtool: parse_mirred.isra.0+0x370: mlx5e_tc_act_vlan_add_push_action() missing __noreturn in .c/.h or NORETURN() in noreturns.h

The error occurs because objtool cannot determine that unreachable BUG()
(which doesn't return) calls in VLAN code paths are actually dead code
when VLAN support is disabled.

Signed-off-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250616132626.1749331-2-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>