openvswitch: Free tmpl with tmpl_free.

When an error occurs during conntrack template creation as part of
actions validation, we need to free the template. Previously we've been
using nf_ct_put() to do this, but nf_ct_tmpl_free() is more appropriate.

Signed-off-by: Joe Stringer <joe@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>