rds: fix building with IPV6=m

When CONFIG_RDS_TCP is built-in and CONFIG_IPV6 is a loadable
module, we get a link error agains the modular ipv6_chk_addr()
function:

net/rds/tcp.o: In function `rds_tcp_laddr_check':
tcp.c:(.text+0x3b2): undefined reference to `ipv6_chk_addr'

This adds back a dependency that forces RDS_TCP to also be
a loadable module when IPV6 is one.

Fixes: e65d4d96 ("rds: Remove IPv6 dependency")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>