netfilter: ip6t_REJECT: fix a dst leak in ipv6 REJECT

We should release dst if dst->error is set.

Bug introduced in 2.6.14 by commit e104411b
([XFRM]: Always release dst_entry on error in xfrm_lookup)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Patrick McHardy <kaber@trash.net>