udp: advertise ipv6 udp support for msghdr::ubuf_info

Mark udp ipv6 as supporting msghdr::ubuf_info. In the original commit
SOCK_SUPPORT_ZC was supposed to be set by a udp_init_sock() call from
udp6_init_sock(), but
d38afeec ("tcp/udp: Call inet6_destroy_sock() in IPv6 ...")
removed it and so ipv6 udp misses the flag.

Cc: <stable@vger.kernel.org> # 6.0
Fixes: e993ffe3 ("net: flag sockets supporting msghdr originated zerocopy")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>