net_sched: reject unknown tcfa_action values

After the commit 802bfb19 ("net/sched: user-space can't set
unknown tcfa_action values"), unknown tcfa_action values are
converted to TC_ACT_UNSPEC, but the common agreement is instead
rejecting such configurations.

This change also introduces a helper to simplify the destruction
of a single action, avoiding code duplication.

v1 -> v2:
 - helper is now static and renamed according to act_* convention
 - updated extack message, according to the new behavior

Fixes: 802bfb19

 ("net/sched: user-space can't set unknown tcfa_action values")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>