ipv6: remove unnecessary codes in tcp_ipv6.c

opt always equals np->opts, so it is meaningless to define opt, and
check if opt does not equal np->opts and then try to free opt.

Signed-off-by: RongQing.Li <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>