net/sched: cls_api: Add extack message for unsupported action offload

For better error reporting to user space, add an extack message when the
requested action does not support offload.

Example:

 # echo 1 > /sys/kernel/tracing/events/netlink/netlink_extack/enable

 # tc filter add dev dummy0 ingress pref 1 proto all matchall skip_sw action nat ingress 192.0.2.1 198.51.100.1
 Error: cls_matchall: Failed to setup flow action.
 We have an error talking to the kernel

 # cat /sys/kernel/tracing/trace_pipe
       tc-181     [000] b..1.    88.406093: netlink_extack: msg=Action does not support offload
       tc-181     [000] .....    88.406108: netlink_extack: msg=cls_matchall: Failed to setup flow action

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>