net: vxlan: check ipv6_mod_enabled() on neigh_reduce()

IPv6 must be enabled or otherwise neigh_reduce() might cause a kernel
panic. This was prevented by a check on in6_dev. Use ipv6_mod_enabled()
instead as it is cleaner and also consistent with the code at
route_shortcircuit().

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260330121033.4479-1-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>