bareudp: use ipv6_mod_enabled to check if IPv6 enabled

bareudp: use ipv6_mod_enabled to check if IPv6 enabled

bareudp_create_sock() use AF_INET6 by default if IPv6 CONFIG enabled.
But if user start kernel with ipv6.disable=1, the bareudp sock will
created failed, which cause the interface open failed even with ethertype
ip. e.g.

 # ip link add bareudp1 type bareudp dstport 2 ethertype ip
 # ip link set bareudp1 up
 RTNETLINK answers: Address family not supported by protocol

Fix it by using ipv6_mod_enabled() to check if IPv6 enabled. There is
no need to check IS_ENABLED(CONFIG_IPV6) as ipv6_mod_enabled() will
return false when CONFIG_IPV6 no enabled in include/linux/ipv6.h.

Reported-by: Jianlin Shi <jishi@redhat.com>
Fixes: 571912c69f0e ("net: UDP tunnel encapsulation module for tunnelling different protocols like MPLS, IP, NSH etc.")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20220315062618.156230-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>