tcp: Fix IPV6 module build errors

If CONFIG_IPV6=m is selected, we are getting following build errors.

net/built-in.o: In function `tcp_is_local6':
net/ipv4/tcp.c:3261: undefined reference to `rt6_lookup'

Making the code conditional upon only CONFIG_IPV6=y fixes this issue.

Also export tcp_nuke_addr to build IPv6 modules. Otherwise
we run into following build error:

  CC [M]  lib/zlib_deflate/deftree.o
  CC [M]  lib/zlib_deflate/deflate_syms.o
  LD [M]  lib/zlib_deflate/zlib_deflate.o
  Building modules, stage 2.
  MODPOST 46 modules
ERROR: "tcp_nuke_addr" [net/ipv6/ipv6.ko] undefined!
make[2]: *** [__modpost] Error 1

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
CC: John Stultz <john.stultz@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>