With NETFILTER_XT_TARGET_TEE=y and IP6_NF_IPTABLES=m, we get a link
error when referencing the NF_DUP_IPV6 module:

net/netfilter/xt_TEE.o: In function `tee_tg6':
xt_TEE.c:(.text+0x14): undefined reference to `nf_dup_ipv6'

The problem here is the 'select NF_DUP_IPV6 if IP6_NF_IPTABLES'
that forces NF_DUP_IPV6 to be =m as well rather than setting it
to =y as was intended here. Adding a soft dependency on
IP6_NF_IPTABLES avoids that broken configuration.

Fixes: 5d400a49 ("netfilter: Kconfig: Change select IPv6 dependencies")
Cc: Máté Eckl <ecklm94@gmail.com>
Cc: Taehee Yoo <ap420073@gmail.com>
Link: https://patchwork.ozlabs.org/patch/999498/
Link: https://lore.kernel.org/patchwork/patch/960062/Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>