net/sched: act_api: fix error code in tcf_ct_flow_table_fill_tuple_ipv6()

The tcf_ct_flow_table_fill_tuple_ipv6() function is supposed to return
false on failure.  It should not return negatives because that means
succes/true.

Fixes: fcb6aa86 ("act_ct: Support GRE offload")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Toshiaki Makita <toshiaki.makita1@gmail.com>
Link: https://lore.kernel.org/r/YpYFnbDxFl6tQ3Bn@kili


Signed-off-by: Paolo Abeni <pabeni@redhat.com>